High speed positive-working photothermographic radiographic film

ABSTRACT

The present invention is directed to a method of forming a positive image in a photothermographic assembly comprising a photothermographic material and an intensifying means for converting ionizing radiation, wherein the assembly has been imagewise exposed to ionizing radiation to form a latent image in the photothermographic material. The photothermographic material has at least one imaging layer comprising a potentially negative-working emulsion, wherein thermal development of unexposed silver salts in exposed areas relative to unexposed areas is inhibiting when thermally developing the imagewise exposed assembly, thereby producing a positive image. The present invention is also directed to a photothermographic assembly that can be used in the present process in which a positive image characterized by high speed and discrimination is formed when exposed and thermally heated above 150° C.

FIELD OF THE INVENTION

This invention relates to a high-speed positive-working silver-halidephotothermographic element for medical and industrial radiography, and aprocess of making an image employing such element.

BACKGROUND OF THE INVENTION

In conventional radiography, films containing light-sensitivesilver-halide grains are employed in a number of image recording devicesincluding but not limited to x-ray imaging cassettes, film baseddosimeters and intra-oral dental film packets. Upon exposure, the filmproduces a latent image that is only revealed after suitable processing.These film elements have historically been processed by treating theexposed film with at least a developing solution having a developingagent that acts to form an image in cooperation with other components inthe film.

It is always desirable to limit the amount of solvent or processingchemicals used in the processing of silver-halide films. The traditionalphotographic processing scheme for black-and-white film involvesdevelopment, fixing and washing, each step typically involving immersionin a tank holding the necessary chemical solution. By the use ofphotothermographic film, it is possible to eliminate processingsolutions altogether, or alternatively, to minimize the amount ofprocessing solutions and the complex chemicals contained therein. Aphotothermographic (PTG) film by definition is a film that requiresenergy, typically heat, to effectuate development. A dryphotothermographic film requires only heat. A solution-minimizedphotothermographic film may require small amounts of aqueous alkalinesolution to effectuate development, which amounts may only be thatrequired to swell the film without excess solution. Development is theprocess whereby silver ion is reduced to metallic silver and in a colorsystem, a dye is created in an image-wise fashion. In manyphotothermographic films, the silver is typically retained in thecoating after the heat development.

In photothermographic films employing what is referred to as “dryphysical development,” a photosensitive catalyst is generally aphotographic-type photosensitive silver halide that is considered to bein catalytic proximity to a non-photosensitive source of reduciblesilver ions. Catalytic proximity requires intimate physical associationof these two components either prior to or during the thermal imagedevelopment process so that when silver atoms, (Ag^(o))_(n), also knownas silver specks, clusters, nuclei, or latent image, are generated byirradiation or light exposure of the photosensitive silver halide, thosesilver atoms are able to catalyze the reduction of the reducible silverions within a catalytic sphere of influence around the silver atoms(Klosterboer, Neblette's Eighth Edition: Imaging Processes andMaterials, Sturge, Walworth & Shepp (eds.), Van Nostrand-Reinhold, NewYork, Chapter 9, pages 279-291, 1989). It has long been understood thatsilver atoms act as a catalyst for the reduction of silver ions, andthat the photosensitive silver halide can be placed in catalyticproximity with the non-photosensitive source of reducible silver ions ina number of different ways (see, for example, Research Disclosure, June1978, item 17029. Research Disclosure is a publication of Kenneth MasonPublications Ltd., Dudley House, 12 North Street, Emsworth, HampshirePO10 7DQ England and also available from Emsworth Design Inc., 147 West24th Street, New York, N.Y. 10011). Research Disclosure, September 1996,Number 389, Item 38957 is hereafter referred to as “Research DisclosureI”.

The non-photo-sensitive source of reducible silver ions is typically amaterial that contains reducible silver ions and preferably a silversalt of an organic compound.

Photothermographic (PTG) media employing dry physical development areformulated with one or more light sensitive imaging layers on a lighttransmitting or reflecting support. Each imaging layer typically has atleast one light-sensitive silver-halide emulsion, a reduciblenon-light-sensitive silver salt, a developer or developer precursor, andoptionally a coupler to form dye. Other components may includeaccelerators, toners, binders, and antifoggants known in the trade aswell as components used in conventional solution-processed silver-halidephotographic media.

When exposed to light and then heated at temperatures ranging from 100to 200° C. for 5 to 60 seconds, photothermographic media developdensities varying with exposure. The density versus log exposure curve(H&D curve) is commonly used in the trade to compare parameters such asspeed and contrast. A typical procedure for generating the H&D curveentails making a contact exposure through a step tablet image. The stepsmodulate the intensity of the incident light, usually in 0.10 to 0.30log exposure increments. Another method entails exposing pixel-wiseusing a laser, CRT or LED source in which the exposure intensity ismodulated electronically. The H&D curve can also be determined usingionizing radiation such as x-radiation that is used in radiography. Onemethod of determining the H&D curve with ionizing radiation is to varythe amount of ionizing radiation received by the photothermographicmedia by passing the ionizing radiation through an ionizing radiationabsorbing step wedge before it impinges on the photothermographicelement. Another method of determining the H&D curve with ionizingradiation is to perform successive exposures of the photothermographicelement at different doses where the amount of ionizing radiationimpinging on the photothermographic media for each exposure isdetermined by using an additional device (such as a dosimeter orradmeter) to measure the amount of ionizing radiation impinging on asurface for a given set of exposure conditions. In both methods theexposed media is processed after exposure then measured.

The measured reflection or transmission density of each step on thephotographic media is then plotted against relative or absolute logexposure to produce what is known in the industry as the “H&D curve.”H&D curves typically have two plateaus corresponding to the maximumdensity (Dmax) and minimum density (Dmin) where the slope of the H&Dcurve approaches or equals zero; that is, a change in exposure produceslittle or no change in measured density. Gamma refers to the slope ofthe H&D curve usually at some fixed density position. Point gamma refersto the change in density between two adjacent exposure positions in aplot of the H&D values. The mid-scale density refers to the densitymidway between Dmax and Dmin plateaus, or (Dmax−Dmin)/2. Thecorresponding exposure is designated the midscale exposure.

As used herein with respect to the present invention, the term“negative-working” refers to a photographic silver-halide emulsion thatdevelops more density with increasing exposure up to the Dmax limit whenan imagewise-exposed gelatin coating of the emulsion is processed usinga solution-development process and concomitant materials in accordancewith the well-known and conventional D-76 standard. The correspondingH&D curve has a positive slope in the mid-scale density range whendensity is plotted against increasing relative log exposure. Theunexposed areas develop to Dmin. The image produced in this way isreferred to as a “negative image.” It is to be understood that the term“negative-working emulsion” as used herein is synonymous with“potentially negative-working emulsion” and refers to an inherentcapability of the emulsion that may or may not be realized in practice.A “positive-working” photographic silver-halide emulsion, as used hereinwith respect to the present invention, responds to exposure bydeveloping less density with increasing exposure down to the saturationlimit (Dmin) when an imagewise-exposed gelatin coating of the emulsionis processed using a solution-development process and materials inaccordance to the well-known D-76 standard. In this case, the H&D curvehas a negative slope in the mid-scale density region when density isplotted against increasing relative log exposure. The unexposed areasdevelop to Dmax. The image produced in this way is referred to as a“positive image.”

Materials, including solution developers, qualifying for commerciallyacceptable use in a D-76 standard process include Kodak's trademarkedproducts designed for such a process. See G. Haist, “Modern PhotographicProcessing, Vol 1”, John Wiley & Sons, Chapter 7, p 340 (1979) for thepreparation of D-76 developer and other related developer formulas, thedisclosure of which is hereby incorporated by reference. D-76 developer,therefore, includes any or all materials designated for and commerciallyused, with commercially satisfactory results in a D-76 process.Preferably, the D-76 developer is a Kodak product or one that issubstantially equivalent in practice.

In a positive-working or negative-working emulsion, the developeddensity can comprise either silver, or if the imaging layer alsocontains a dye-forming coupler to react with oxidized developer, silverplus dye.

In the case of conventional solution-processed photographic media, ascompared to dry or apparently dry thermally developed photothermographicmedia, positive images can be obtained from negative-working emulsionsusing combinations of multiple exposures and/or multiple developmentsteps. See G. Haist, cited above, for details on black-and-white andcolor reversal-development processes, in which the following patents arecited: U.S. Pat. No. 2,005,837, U.S. Pat. No. 2,126,516, U.S. Pat. No.2,184,013, U.S. Pat. No. 2,699,515, U.S. Pat. No. 3,361,564, U.S. Pat.No. 3,367,778, U.S. Pat. Nos. 3,455,235, 3,501,310, U.S. Pat. No.3,519,428, U.S. Pat. No. 3,560,213, U.S. Pat. No. 3,579,345, U.S. Pat.No. 3,650,758, U.S. Pat. No. 3,655,390, BR 44248, BR 1151782, BR1155404, BR 1186711, BR 1201792, CA 872180, and CA 872181.

For example, photobleach emulsions can be used in conventionalsolution-developed silver-halide photographic media to produce positiveimages. These emulsions are prepared with desensitizing dyes andchemical fogging agents. An exposure destroys preformed surface fogcenters rendering the grains undevelopable. The unexposed grains developto form a positive image. G. Haist reviews this topic in ModernPhotographic Processing, Vol 2, Chapter 7, John Wiley & Sons, (copyright1979).

GB 2018453A to Willis et al. teaches a photothermographic elementcomprising resorcinolic coupler, phenylenediamine developer, gelatin,silver bromoiodide emulsion (negative-working), various reducibleorganic silver salts (notably the silver salt of3-amino-5-benzylthio-1,2,4-triazole (ABT)), and an antifoggant3-methyl-5-mercapto-1,2,4-triazole (MMT). Slusarek et al., in U.S. Pat.No. 6,319,640 and U.S. Pat. No. 6,312,879, describes blockedphenylenediamine developers for photothermographic media coated fromwater and gelatin.

Negative-working photographic silver-halide emulsions are used inradiography for both industrial and medical applications.Negative-working photographic silver-halide emulsions can be used toimage ionizing radiation directly or indirectly by the use of anintensifying element for ionizing radiation. An intensifying element isused for converting ionizing radiation to a lower-energy form suitablefor exposing photographic or photothermographic elements. Inradiography, intensifying elements are used in conjunction withphotographic elements. Known intensifying elements include, for example,inorganic and organic phosphors as well as metal particles and metalfoils. Intensifying elements in radiography can also be intensifyingscreens, imaging plates, radiographic screens, or phosphor screens. Mostintensifying screens used in radiography contain luminescent materialscalled phosphors, scintillators, or luminophores. These materials, oftenin the form of particles, emit visible light upon irradiation withionizing radiation. The light emitted by the phosphor leaves theintensifying element or screen and impinges on the negative-workingphotographic silver-halide emulsion to form the latent image that issubsequently developed imagewise.

Thus, radiographic films can be used in combination with some othermaterial to convert the x radiation to another radiation form that canbe more readily detected by silver halide in the films. Such radiationconverting materials can be metal plates of metal oxides that convertx-radiation to electrons or can be inorganic phosphors that convertx-radiation to visible radiation. Such converting materials are usuallyprovided in a separate element in what is known as “metal screens,”intensifying screens, or phosphor panels. If phosphors or metal oxidesare included within the typical silver halide emulsion, image noiselevels may increase. This is due to the fact that electrons or visibleradiation from the converting materials may expose silver halide grainsoutside of the image area, giving rise to image noise. Thus metal orphosphor intensifying screens or panels may be preferred for use incombination with radiographic films in what are known as cassettes orradiographic imaging assemblies. However, the incorporation of phosphorsin silver-halide emulsions are known. U.S. Pat. No. 6,440,944 teachesthe use of negative imaging photothermographic elements withintensifying screens as well as the direct addition of x-ray sensitivephosphors to the photothermographic element to prepare a radiographicelement suitable for imaging.

Negative-working photothermographic silver-halide emulsions are used inmedical imaging as image-receiving elements in laser printing stationssuch as the KODAK DryView® laser printer. These laser printer stationsare used to obtain hard copy images, of results from physicalexaminations of patients, taken using digital imaging modalities such asmagnetic resonance imaging, ultrasound, positron emission tomography,computer aided tomography, and computed radiography. Thenegative-working photothermographic silver-halide emulsions used inthese systems is not exposed to ionizing radiation. An example of anegative-working photothermographic silver-halide emulsion that can beused to image ionizing radiation is described in U.S. Pat. No. 6,440,649by Simpson et al.

Historically, photographic films containing various silver halides havebeen used for various radiographic purposes. Such films have exhibitedexcellent sensitivity to x-radiation, high spatial resolution, low imagenoise, and archival storage properties. Desired sensitivity to imagingx-radiation has been achieved through amplification of a relativelysmall number of latent image centers without too much noise being addedto the image. The term noise is understood in radiography to refer tothe random variations in optical density throughout a radiographic imagethat impairs the user's ability to distinguish objects within the image.Radiographic noise is considered to have a number of componentsidentified in the art as quantum mottle, film grain, and structuremottle as noted, for example, by Ter-Pogossian, THE PHYSICAL ASPECTS OFDIAGNOSTIC RADIOLOGY, Harper and Row, New York, Chapter 7, 1967.

Positive-working photographic silver-halide emulsions are not generallyused for imaging ionizing radiation or in radiography. There are noknown positive-working photothermographic silver-halide emulsions thatare sensitive to ionizing radiation.

A significant problem with photothermographic elements has been thedifficulty obtaining high photographic speeds. Silver-halide emulsionsthat are optimally sensitized for photographic speed in aqueous gelatingenerally lose speed in contact with organic solvents and non-gelatinbinders that are used in many non-aqueous photothermographic systems.Organic solvents may induce dye desorption, dye deaggregation, or someother chemical effect that degrades photographic efficiency. Methods ofchemical and spectral sensitizations in organic solvents are lesseffective than in water for similar reasons.

Gelatin coatings, on the other hand, are more difficult to thermallydevelop due to the physical properties of the gelatin when it is heated.Lower developed density and photographic speed generally result from thehigher glass transition temperature of gelatin and generally slowerrates of diffusion of developer components in the strong hydrogenbonding polypeptide matrix. Gelatin coatings also require dispersing theincorporated water-insoluble developer components, which causes them toreact generally more sluggishly under thermal processing conditionscompared to organic solvent coatings in which developer components aredissolved in the coating solvent.

In addition, all of the prior art describes photothermographic systemsthat produce negative images that are nearly equal in speed to thoseobtained with solution development. In contrast, the present inventioncan produce direct-positive photographic speeds that are two to threestops greater than speeds obtained by solution or thermal development ofsame-size negative-working silver-halide emulsions.

SUMMARY OF THE INVENTION

The present invention is directed to a method of using ionizingradiation to form an image in a positive-working photothermographicelement or material, such as film, comprising a potentiallynegative-working emulsion but in which fog-density development inexposed areas of the image is imagewise inhibited upon thermaldevelopment. By “fog density” is meant the thermal development, in theemulsion, of unexposed silver particles, whether light-sensitive and/ornon-light sensitive silver-containing particles. The image can bemonochrome or bichrome. Without wishing to be bound by theory, it isbelieved that imagewise inhibition occurs by the presence of aninhibiting agent or precursor thereof, for example, aninhibitor-releasing compound that releases a density inhibitor uponthermal development.

Preferably, the method of exposure first entails making a light-tightfilm packet containing the photothermographic element in combinationwith one or more intensifying means, which intensifying means can be,for example, phosphor intensifying screens, phosphors incorporatedwithin the photothermographic emulsion, or with a combination thereof.The intensifying means, if not incorporated within the emulsion, canstill be a unitary or integral part of the photothermographic element.In this case, the photothermographic element or material can beoptionally exposed directly to ionizing radiation. By “unitary” or“integral” is meant that the intensifying means is not readily separablefrom the photothermographic material comprising the silver-halideemulsion and is, therefore, is adapted for being subjected to thermaldevelopment together with the photothermographic material. For example,the photothermographic element can comprise a multi-layer structurecomprising a coated layer of an intensifying means and a coated imaginglayer. Alternatively, an intensifying means can be an intensifyingscreen or the like adapted for physical separation from thephotothermographic element in which case the intensifying element andthe photothermographic element are employed in combination.

In any case, the combination of the photothermographic material,comprising an imaging layer, and the intensifying means will be referredto herein as an imaging or photothermographic “assembly” irrespective ofwhether the intensifying means is part of the developable material orseparable therefrom. Unless otherwise indicated, a photothermographic“element” is a photothermographic material that may optionally comprisean incorporated intensifying means and/or which may be used in anassembly that comprises a separable intensifying element and thermallydeveloped photothermographic element. Thus, the photothermographicelement can be a photothermographic assembly or part of one.

In one embodiment of the invention, for example, the photothermographicmaterial is exposed by visible-light emitting phosphor particles coatedon a support, which support is separate from the support for thesilver-halide emulsion in the photothermographic material, to form anexternal phosphor screen. The phosphor particles emit light in responseto impinging ionizing radiation. The method of exposure first entailsmaking a light-tight film packet of the photothermographic element andthe external screen. In the film packet, the photothermographic elementand phosphor-containing layer of the external screen are in face-to-facecontact. The object to be imaged is then placed between the film packetand the source of ionizing radiation. The object modulates the x-rayexposure to produce an x-ray transmission image that penetrates the filmpacket and is absorbed by the external screen. The external screenconverts the x-ray energy into visible-light energy that is recorded bythe photothermographic element in the form of a latent silver image.During thermal development of the photothermographic element, adensity-inhibiting agent inhibits the thermal (fog-density) developmentof unexposed silver particles (density) in the exposed areas relative tothe unexposed areas of the element to produce a positive image in thephotothermographic film.

In another embodiment, the photothermographic element comprises in partan internal phosphor, that is internal to, and integral with, thephotothermographic element. The internal phosphor performs the samefunction as the external phosphor screen but it is coated within thephotothermographic element. The internal phosphor may occupy the samelayer as the silver halide emulsion or can be in a nearby layer of thephotothermographic element. An internal phosphor may preclude the needfor an external phosphor screen. In a preferred embodiment, one or morecouplers or the like is present in the photothermographic element toaccelerate development by removing Dox as it is formed, in order todrive development to Dmax.

Without wishing to be bound by theory, it is believed that thermaldevelopment in the present invention comprises (in order) two stages: afirst stage comprising amplification of the latent image to form arelatively low-contrast negative image; and a second stage comprisingimagewise inhibition of fog development (by an agent released by aninhibitor-releasing compound) to form a final relatively high-contrastpositive image.

The present invention is also directed to a photothermographic elementthat can be used in the present process.

The present invention has the advantage of high speeds. In fact, theabove-mentioned second-stage positive image, taken to full developmentin the unexposed areas, can be at least two stops faster than thefirst-stage negative image. Thus, the inventive method and accompanyingphotothermographic element can form a positive image of high speed anddiscrimination when exposed and heated 10 to 40 sec at 150 to 185° C.Images have excellent thermal and light stability. Dmins (minimumdensities) are stable after extended incubation to heat or light. Theseand other advantages will be apparent from the detailed descriptionbelow.

Definitions of other terms, as used herein, include the following:

In the descriptions of the photothermographic materials of the presentinvention, “a” or “an” component refers to “at least one” of thatcomponent.

Heating in a substantially water-free condition as used herein, meansheating at a temperature of from about 150° C. to about 200° C. withlittle more than ambient water vapor present. The term “substantiallywater-free condition” means that the reaction system is approximately inequilibrium with water in the air, and water for inducing or promotingthe reaction is not particularly or positively supplied from theexterior to the material. Such a condition is described in T. H. James,The Theory of the Photographic Process, Fourth Edition, Macmillan 1977,p 374.

“Emulsion layer,” “imaging layer,” or “photothermographic emulsionlayer,” means a layer of a photothermographic material that contains thephotosensitive silver halide (when used) and non-photosensitive sourceof reducible silver ions.

“Non-photosensitive” means not intentionally light sensitive.

The term “organic silver salt” is herein meant to include salts as wellas ligands comprising two ionized species. The silver salts used arepreferably comprised of silver salts of organic coordinating ligands.Many examples of such organic coordinating ligands are described below.The silver donors can comprise asymmetrical silver donors or dimers suchas disclosed in commonly assigned U.S. Pat. No. 5,466,804 to Whitcomb etal. In the case of such dimers, they are considered to be two separateorganic silver salts such that only one silver atom is attributed toeach organic silver salt. Organic silver salts can be in the form ofcore-shell particles as disclosed in commonly assigned U.S. Pat. No.6,548,236.

The terms “blocked developer” and “developer precursor” are the same andare meant to include developer precursors, blocked developer, hindereddevelopers, and developers with blocking and/or timing groups, whereinthe term “developer” is used to indicate a reducing substance for silverion. The term “image” and “imagewise” broadly refers, in one case, toany image or visual representation, including a picture, indicia, print,symbol, or positive indication or readout, including reproductionscharacterized by photographic-quality images as well asinformation-providing representations, including measurement indicatorsor signifiers such as a radiation dosimeter.

“Ultraviolet region of the spectrum” refers to that region of thespectrum less than or equal to 410 nm and preferably from about 100 nmto about 410 nm, although parts of these ranges may be visible to thenaked human eye. More preferably, the ultraviolet region of the spectrumis the region from about 190 nm to about 405 nm.

“Short wavelength visible region of the spectrum” refers to that regionof the spectrum from about 400 nm to about 450 nm.

“Visible region of the spectrum” refers to that region of the spectrumfrom about 400 nm to about 700 nm.

“Red region of the spectrum” refers to that region of the spectrum fromabout 600 nm to about 700 nm.

“Infrared region of the spectrum” refers to that region of the spectrumfrom about 700 nm to about 1400 nm.

“Middle chalcogen” means sulfur (S), selenium (Se), or tellurium (Te).

“Non-photosensitive” means not intentionally light sensitive.

“Transparent” means capable of transmitting visible light or imagingradiation without appreciable scattering or absorption.

A “Phosphor” is an organic or inorganic compound that is responsive toionizing radiation and more preferably x-radiation and, uponirradiation, emits radiation in the ultraviolet, visible, or infraredregion of the spectrum. Most phosphors emit such radiation immediatelyupon exposure to stimulating radiation. However, some phosphors areknown as “storage” phosphors because they have the capacity to storeenergy from the initial irradiation and to release the light at a latertime when stimulated by still other radiation. Another class ofphosphors are known as “thermoluminescent” phosphors because they havethe capacity to store energy from the initial irradiation and to releasethe light at a later time when stimulated by heat.

The term “kVp” and “MVp” stand for peak voltage applied to an x-ray tubetimes 10³ and 10⁶ respectively. The term “mA” stands for themilliamperes of current applied between the anode and the cathode of thex-ray tube during exposure. The term “dose” refers to the exposurereceived by a given object when irradiated. Dose is measured either inunits of “Rads” (1 Rad is the energy absorption of 100 ergs per gram oftissue) or the dose equivalent units known as “Rems.” “mRems” stands for“Rems” times 10^(−3.)

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph (blue transmission density vs relative log exposure)showing the effect of different external phosphor screens on thephotographic H&D curve for one embodiment of a photothermographic filmaccording to the present invention as described in Example 2 below.

DETAILED DESCRIPTION OF THE INVENTION

According to the method of the present invention, a positive image isformed in a photothermographic element (such as film), comprising apotentially negative-working emulsion, by employing aninhibitor-releasing compound that imagewise inhibits fog-densitydevelopment in exposed areas of the image during thermal development,which photothermographic element comprises, or is used in combinationwith, an intensifying means. The intensifying means is an element ormaterial that converts ionizing radiation into a form that is useful forimagewise exposing the photothermographic element resulting in theformation of a latent image. The combination of the photothermographicmaterial, comprising an imaging layer, and the intensifying means willbe referred to herein as an imaging or photothermographic “assembly”irrespective of whether the intensifying means is part of thedevelopable material or separable therefrom. Unless otherwise indicated,a photothermographic “element” is a photothermographic material that mayoptionally comprise an incorporated intensifying means and/or which maybe used in an assembly that comprises a separable intensifying elementand thermally developed photothermographic element. Thus, thephotothermographic element can be a photothermographic assembly or partof one.

According to the method of the present invention, thermal development ofunexposed silver salts in the exposed areas is inhibited relative to theunexposed areas, with the proviso that the element is imagewise exposedwith a non-solarizing amount of ionizing radiation or a non-solarizingamount of actinic radiation emitted from an intensifying means inresponse to excitation from ionizing radiation in order to form a latentimage, and the latent image is thermally developed in a singledevelopment step, without any reversal steps or additional exposures toactinic radiation, to produce a positive image in the film. Theabove-mentioned inhibition is believed to be caused by andensity-inhibiting agent that may be present or released during thermaldevelopment, for example, released by a density-inhibitor-releasingcompound (as in believed to occur in a preferred embodiment) but, in anycase, the key is that inhibition is accomplished.

In another aspect of the present invention, a photothermographicelement, comprising at least one image-forming layer coated on asupport, said layer comprising at least one photographically activesilver-halide emulsion spectrally sensitized to visible light and atleast one non-light-sensitive organic silver salt), following imagewiseexposure to actinic radiation from an intensifying means which in turnis exposed to ionizing radiations, is developed by heating at 150-200°C., to develop an imagewise reduced-silver image that is physicallyseparate and morphologically distinct from the developed latent-imagesilver associated with the silver-halide grains. In one preferredembodiment, the photothermographic element comprises at least onenon-light sensitive organic silver salt which releases theinhibitor-releasing compound.

The present invention involves forming a high-speed, stable positiveimage when a photothermographic material is thermally developed in thepresence or absence of the intensifying means for converting ionizingradiation. In the preferred embodiment, at least one imaging layercomprises a negative-working silver-halide emulsion, at least onenon-light sensitive silver salt, an inhibitor-releasing compound, adeveloper or precursor thereof, and preferably a scavenging agent forthe oxidized developer Dox.

In one preferred embodiment, for example, at least one imaging layercomprises a negative-working silver halide emulsion, at least onenon-light-sensitive silver salt which functions as aninhibitor-releasing compound, a blocked phenylenediamine developer, aphenolic developer/coupler, and a thermal solvent, for example, ahydroxy-substituted benzamide. One may also incorporate optional tonersand accelerators known in the trade, examples of which includesuccinimide, phthalimide, naphthalimide, phthalazine, and phthalazinone.Other components that can be used are described in U.S. PatentPublication 2004/0033447 A1, hereby incorporated by reference it itsentirety.

The intensifying means for converting ionizing radiation may be avisible-light emitting phosphor in the imaging layer or adjacent layersor a metal foil in adjacent layers. After exposure to ionizing radiationthe photothermographic emulsion develops a positive image when theexposed invention element is heated at a temperature of at least 150° C.for at least 20 sec, preferably at least 155° C. for at least 20 sec,most preferably 160° C. for 20 to 40 sec. Images can be formed havingexcellent discrimination and are resistant to print out. To Applicants'knowledge, this is the first example of photothermographic elementincorporating a negative-working emulsion that develops a positive imagewhen given a non-solarizing exposure of actinic radiation from anintensifying means in response to ionizing radiation, in the absence ofmultiple development steps as in reversal development. In contrast, asolarizing exposure is an extended exposure beyond the level required toproduce a stable latent image. Less density develops in this casebecause the extended exposure causes the release of sufficient halogento reoxidize the latent image. By the phrase “absence of multipledevelopment steps” is meant that development occurs in a singleunit-process step. Full development can occur during a heating stepwherein once the film is heated to initiate development the developmentis complete before bringing the film back to temperature below whichthermal development is initiated. For example, in one embodiment, thedevelopment is initiated above 150° C. and completed before bringing thetemperature below 150° C. There are no separate reversal steps, orreexposures of the photographic element, for complete development.Instead, thermal development, involving both a relatively low-contrastnegative image and its change to a final positive image, occurs in asingle or continuous heating step.

Without wishing to be bound by theory, the Applicants believe thefollowing events occur during the present process. In an initial stageof thermal development, latent image amplification occurs in the normalsense to produce a low-contrast negative image. During this initialstage, a development inhibitor is released. The inhibitor is believed toshut down negative-image development shortly after initiation. In asecond stage of thermal development, in which unexposed silver halideand non-light-sensitive silver salts are thermally developed or reducedto silver (referred to as “fogging”) at sufficiently high temperature,the developed density in the initial negative-image development stagebecomes the Dmin of the final positive image. A coupler, if present, mayreact with oxidized developer to form a negative image consisting of dyeplus silver. Colors can appear quite saturated in the negative image.With continued heating the exposed areas resist further developmentwhile the unexposed areas rapidly develop to a high-density fog.

If a coupler is present, the hue may appear less saturated in theunexposed areas. The result is a positive two-toned image possessinghigh speed and excellent light stability, suitable for scanning or, insome cases, for direct viewing.

Electron micrographs reveal that, during the second stage of thermaldevelopment, some of the silver development can occur off-grain and mayinvolve the photographically inactive non-halide silver ion donorsduring dry physical development. Increasing exposure of thenegative-working photosensitive silver halide grains results in lessoff-grain silver development. This provides the advantage of increasedcovering power and developed density in the areas of least exposure.

Without wishing to be bound by theory, the Applicants postulate thatpositive-image development occurs via formation of a sphere ofinhibition around the exposed and partially developed negative-workingsilver-halide grains.

In a preferred embodiment, two different silver ion donors are presentin the imaging layer, one or both of which release a development ordensity-inhibiting agent. However, other sources of the developmentinhibitor can be used, for example, as a PUG (photographically usefulgroup) that is releasable from a coupler or other compound present inthe imaging layer. For example, in one embodiment of the invention,phenylmercaptotetrazole (PMT) or benzotriazole, two known developmentinhibitors commonly used in the trade to make DIR couplers(development-inhibitor-releasing couplers), are believed to accumulateduring the initial stage of dry physical development in the vicinity ofthe partially amplified negative image, when only the latent imagedevelops. It is postulated that at a critical concentration, theinhibitor shuts down further latent-image development and also slows therate of fog formation or development in the exposed areas. The unexposedareas appear to produce fog at a normally high kinetic rate, fast enoughto develop to a high density before released inhibitor can shut downdevelopment. The result is a positive image having high discriminationand speed.

In a preferred embodiment, the photographic speed of a givennegative-working emulsion in the dry reversal coating format is 2-3stops higher in photographic speed compared to conventionalsolution-processed or thermal-processed coatings that produce a negativeimage. Images are quite stable to extended exposure to light.

In one embodiment of the invention, in which the photographic elementcomprises two organic silver salts, the first organic silver saltexhibits a pKsp difference of at least 0.5, preferably at least 1.0,more preferably at least 2.0 less than the pKsp of the second organicsilver salt or ligand. In one particularly preferred embodiment, thefirst organic silver ligand exhibits a cLogP of 0.1 to 10 and a pKsp of7 to 14 and the second organic silver ligand exhibits a cLogP of 0.1 to10 and a pKsp of 14 to 21. In another embodiment, the first organicsilver salt, or salt of the first type, has a pKsp of 9 to 16 and thesecond organic silver salt, or the organic silver salt of the secondtype, has a pKsp of 12 to 19.

In another embodiment, the organic ligands used to make the first andsecond silver salts are combined together to form a single mixed silversalt of various molar compositions.

When individual organic silver salts are used, both organic silver saltsare present at levels above 5 g/mol of imaging silver halide.Preferably, the first organic silver salt is primarily the silver donorduring the initial stage of thermal development (or the more reactivesilver donor), at levels in the range of 5 to 3,000 g/mol of imagingsilver halide. Preferably, the second organic silver salt acts as thethermal fog inhibitor, in the first stage of thermal development, and ispresent at levels in the range of 5 to 3,000 g/mol of imaging silverhalide. Preferably, molar ratio of said first organic silver salt tosaid second organic silver salt is from about 0.1:10 to about 10:1.

In a preferred embodiment of the present invention, a photothermographicelement has on a support one or more one light-sensitive imaging layers,each of said imaging layers comprising a light-sensitive silveremulsion, a binder, a dye-providing coupler or other Dox scavenger, anda developer or blocked developer. Preferably, the dyes or othercompounds formed from the Dox scavenger in the layers are capable offorming a dye image of a visible or non-visible color. By the term“visible or non-visible colors” is meant that colorless compounds mayabsorb light outside the visible wavelength region (400-700 nm).

Although the minimum value of the indicated difference in pKsp is 0.5,preferably the difference in pKsp is at least 1.0, more preferably atleast 2.0. The lower the temperature onset, however, the less thedifference in pKsp that is needed. In one embodiment of the invention,both the first and second organic silver salt, or both the first andsecond type of organic silver salt, have a pKsp of greater than 11,preferably greater than 12, and neither are silver carboxylates,including silver behenate.

The activity solubility product or pK_(sp) of an organic silver salt isa measure of its solubility in water. Some organic silver salts are onlysparingly soluble and their solubility products are disclosed, forexample, in Chapter 1 pages 7-10 of The Theory of the PhotographicProcess, by T. H. James, Macmillan Publishing Co. Inc., New Your (fourthedition 1977). Many of the organic silver salts consist of thereplacement of a ligand proton with Ag+. The silver salts derived frommercapto compounds are relatively less soluble. The compound PMT has apK_(sp) of 16.2 at 25° C. as reported by Z. C. H. Tan et al., Anal.Chem., 44, 411 (1972); Z. C. H. Tan, Phototgr. Sci. Eng., 19, 17 (1975).In comparison, benzotriazole, for example, has a pKsp of 13.5 at atemperature of 25° C. as reported by C. J. Battaglia, Photogr. Sci.Eng., 14, 275 (1970).

In a preferred embodiment, the primary source of reducible,non-photosensitive silver in the practice of this invention are organicsilver salts described as having the lower pKsp.

The first organic silver salt, or first type of organic silver salt, ispreferably a non-photosensitive source of reducible silver ions (thatis, silver salts) and can be any compound that contains reducible silver(1+) ions. Preferably, it is a silver salt that is comparatively stableto light and forms a silver image when heated to 50° C. or higher in thepresence of an exposed photocatalyst (such as silver halide) and areducing composition. In the imaging layer of the element, thephotocatalyst and the non-photosensitive source of reducible silver ionsmust be in catalytic proximity (that is, reactive association).“Catalytic proximity” or “reactive association” means that they shouldbe in the same layer, or in adjacent layers. It is preferred that thesereactive components be present in the same emulsion layer.

According to the present invention, the organic silver salt referred toas the “organic silver donor” or “the first organic silver salt” or“organic silver salt of the first type” is generally the oxidativelymore reactive organic silver salt compared to the second organic silversalt or second type of organic silver salt. This more reactive organicsilver salt is preferably a silver salt of a nitrogen acid (imine)group, which can optionally be part of the ring structure of aheterocyclic compound. Aliphatic and aromatic carboxylic acids such assilver behenate or silver benzoate, in which the silver is associatedwith the carboxylic acid moiety, are specifically excluded as theorganic silver donor compound. Compounds that have both a nitrogen acidmoiety and carboxylic acid moiety are included as donors of thisinvention only insofar as the silver ion is associated with the nitrogenacid rather than the carboxylic acid group. The donor can also contain amercapto residue, provided that the sulfur does not bind silver toostrongly, and is preferably not a thiol or thione compound.

More preferably, a silver salt of a compound containing an imino grouppresent in a heterocyclic nucleus can be used. Typical preferredheterocyclic nuclei include triazole, oxazole, thiazole, thiazoline,imidazoline, imidazole, diazole, pyridine and triazine. Examples of thefirst organic silver salt include derivatives of a tetrazole. Specificexamples include but are not limited to 1H-tetrazole,5-ethyl-1H-tetrazole, 5-amino-1H-tetrazole,5-4′methoxyphenyl-1H-tetrazole, and 5-4′carboxyphenyl-1H-tetrazole.

The organic silver salt may also be a derivative of an imidazole.Specific examples include but are not limited to benzimidazole,5-methyl-benzimidazole, imidazole, 2-methyl-benzimidazole, and2-methyl-5-nitro-benzimidazole. The organic silver salt may also be aderivative of a pyrazole. Specific examples include but are not limitedto pyrazole, 3,4-methyl-pyrazole, and 3-phenyl-pyrazole.

The organic silver salt may also be a derivative of a triazole. Specificexamples include but are not limited to benzotriazole, 1H-1,2,4-trazole,3-amino-1,2,4 triazole, 3-amino-5-benzylmercapto-1,2,4-triazole,5,6-dimethyl benzotriazole, 5-chloro benzotriazole, and4-nitro-6-chloro-benzotriazole.

Other silver salts of nitrogen acids may also be used. Examples wouldinclude but not be limited to o-benzoic sulfimide,4-hydroxy-6-methyl-1,3,3A,7-tetraazaindene,4-hydroxy-6-methyl-1,2,3,3A,7-pentaazaindene, urazole, and4-hydroxy-5-bromo-6-methyl-1,2,3,3A,7-pentaazaindene.

Most preferred examples of the organic silver donor compounds includethe silver salts of benzotriazole, triazole, and derivatives thereof, asmentioned above and also described in Japanese patent publications30270/69 and 18146/70, for example a silver salt of benzotriazole ormethylbenzotriazole, etc., a silver salt of a halogen substitutedbenzotriazole, such as a silver salt of 5-chlorobenzotriazole, etc., asilver salt of 1,2,4-triazole, a silver salt of3-amino-5-mercaptobenzyl-1,2,4-triazole, a silver salt of 1H-tetrazoleas described in U.S. Pat. No. 4,220,709.

Silver salt complexes may be prepared by mixture of aqueous solutions ofa silver ionic species, such as silver nitrate, and a solution of theorganic ligand to be complexed with silver. The mixture process may takeany convenient form, including those employed in the process of silverhalide precipitation. A stabilizer may be used to avoid flocculation ofthe silver complex particles. The stabilizer may be any of thosematerials known to be useful in the photographic art, such as, but notlimited to, gelatin, polyvinyl alcohol or polymeric or monomericsurfactants.

The photosensitive silver halide grains and the organic silver salt arecoated so that they are in catalytic proximity during development. Theycan be coated in contiguous layers, but are preferably mixed prior tocoating. Conventional mixing techniques are illustrated by ResearchDisclosure, Item 17029 (June 1978), as well as U.S. Pat. No. 3,700,458and published Japanese patent applications Nos. 32928/75, 13224/74,17216/75 and 42729/76.

Preferably, at least one organic silver donor is selected from one ofthe above-described compounds.

In a preferred embodiment, an oxidatively less reactive silver salt (the“second organic silver salt” or organic silver salt of the second type”)is selected from silver salts of thiol or thione substituted compoundshaving a heterocyclic nucleus containing 5 or 6 ring atoms, at least oneof which is nitrogen, with other ring atoms including carbon and up totwo heteroatoms selected from among oxygen, sulfur and nitrogen arespecifically contemplated. Typical preferred heterocyclic nuclei includetriazole, oxazole, thiazole, thiazoline, imidazoline, imidazole,diazole, pyridine and triazine. Preferred examples of these heterocycliccompounds include a silver salt of 2-mercaptobenzimidazole, a silversalt of 2-mercapto-5-aminothiadiazole, a silver salt of5-carboxylic-1-methyl-2-phenyl-4-thiopyridine, a silver salt ofmercaptotriazine, a silver salt of 2-mercaptobenzoxazole. These silversalts are herein referred to as “oxidatively less reactive silversalts.”

The oxidatively less reactive silver salt may be a derivative of athionamide. Specific examples would include but not be limited to thesilver salts of 6-chloro-2-mercapto benzothiazole, 2-mercapto-thiazole,naptho(1,2-d)thiazole-2(1H)-thione, 4-methyl-4-thiazoline-2-thione,2-thiazolidinethione, 4,5-dimethyl-4-thiazoline-2-thione,4-methyl-5-carboxy-4-thiazoline-2-thione, and3-(2-carboxyethyl)-4-methyl-4-thiazoline-2-thione.

Preferably, the oxidatively less reactive silver salt is a derivative ofa mercapto-triazole. Specific examples would include, but not be limitedto, a silver salt of 3-mercapto-4-phenyl-1,2,4 triazole and a silversalt of 3-mercapto-1,2,4-triazole.

Most preferably the oxidatively less reactive silver salt is aderivative of a mercapto-tetrazole. In one preferred embodiment, amercapto-tetrazole compound useful in the present invention isrepresented by the following structure:

wherein n is 0 or 1, and R is independently selected from the groupconsisting of substituted or unsubstituted alkyl, aralkyl, or aryl.Substituents include, but are not limited to, C1 to C6 alkyl, nitro,halogen, and the like, which substituents do not adversely affect thethermal fog inhibiting effect of the silver salt. Preferably, n is 1 andR is an alkyl having 1 to 16 carbon atoms or a substituted orunsubstituted phenyl group. Specific examples include but are notlimited to silver salts of 1-phenyl-5-mercapto-tetrazole,1-(3-acetamido)-5-mercapto-tetrazole, or1-[3-(2-sulfo)benzamidophenyl]-5-mercapto-tetrazole.

In one embodiment of the invention, a first organic silver salt is abenzotriazole or derivative thereof and a second organic silver salt isa mercapto-functional compound, preferably mercapto-heterocycliccompound. Particularly preferred is 1-phenyl-5-mercapto-tetrazole (PMT).

In general, an organic silver salt is formed by mixing silver nitrateand other salts with the free base of the organic ligand such as PMT. Byraising the pH sufficiently with alkaline base, the silver salt of PMTcan be precipitated, typically in spheroids 20 nm in diameter andlarger.

In a particularly preferred embodiment, the photothermographic elementcomprises at least one image forming layer coated on a support, whereinsaid layer comprises at least one silver halide emulsion, optionallychemically and spectrally sensitized to visible or infrared radiation,an organic silver salt having Structure (I), a silver salt havingStructure (II) below, an optional thermal solvent selected fromStructures (IIIA-IIIC), a phenolic coupler of Structure (IV) below, andan amine developer or precursor thereof having Structure (V) below. Suchan element is capable of producing a positive image after a singleexposure and single thermal development unit step.

The silver salt of Structure (I) has the general structure:

wherein R¹ is alkyl, cycloalkyl, substituted alkyl, phenyl, aryl,substituted aryl or phenyl.

The silver salt of Structure (II) has the general structure:

wherein R², R³, R⁴, and R⁵ may be independently selected from hydrogen,halide, alkyl, alkoxy, aryl, phenyl, phenoxy, carboxy, alkyl,cycloalkyl, substituted alkyl, substituted aryl, substituted phenyl,wherein said substituted alkyl, aryl or phenyl groups may also containO, N, S, halide, sulfonic acid, sulfone, sulfonamide, carboxylic acid,ester, aldehyde, ketone, amine, or amide; and wherein at least two ofR², R³, R⁴, and R⁵ may be part of an additional ring structure.

In another embodiment mixed silver salts of the organic ligands used tomake Structure (I) and Structure (II) may be preferred over theindividual salts. An example is a mixed salt comprising silver,benzotriazole, and PMT in the molar ratio of 1:0.5:0.5. Prior artthermal solvents for a heat processed photographic elements aredisclosed in U.S. Pat. No. 6,277,537, U.S. Pat. No. 5,436,109; U.S. Pat.No. 5,843,618, U.S. Pat. No. 5,480,761, U.S. Pat. No. 5,480,760, U.S.Pat. No. 5,468,587, U.S. Pat. No. 5,352,561, U.S. Pat. No. 5,064,742.These are also useful in the current invention although optional. Whenused, preferred thermal solvents have a hydroxy-benzamide structure asshown in Structures (IIIA)-(IIIC):

wherein R¹¹, R¹², R¹³, R¹⁴, R¹⁵, and R¹⁶, which can be the same ordifferent individually, can be hydrogen, alkyl, substituted alkyl,alkenyl, substituted alkenyl, aryl, substituted aryl, halogen, cyano,alkoxy, substituted alkoxy, aryloxy, substituted aryloxy, amino,substituted amino, alkylcarbonamido, substituted alkylcarbonamido,arylcarbonamido, substituted arylcarbonamido, alkylsulfonamido,arylsulfonamido, substituted alkylsulfonamido, substitutedarylsulfonamido, or sulfamyl; or wherein at least two of R¹¹, R¹², R¹³,R¹⁴, R¹⁵, and R¹⁶ together can further form a substituted orunsubstituted carbocyclic or heterocyclic ring structure that canfurther be substituted or unsubstituted.

Representative thermal solvents include:

The phenolic coupler of Structure (IV) has the general structure:

wherein R⁶, R⁷, R⁸, R⁹ and R¹⁰ may independently be selected fromhydrogen, hydroxyl, alkyl, alkoxy,

wherein R²⁰, R²¹, R²², R²³ are independently selected from alkyl,haloalkyl, hydroxyl, amino, substituted amino, arylamino, substitutedarylamino, aryl, substituted aryl, phenyl, substituted phenyl, alkoxy,aryloxy, substituted aryloxy, phenoxy, and substituted phenoxy, orwherein at least two of R⁷, R⁸, and R⁹ together can further form asubstituted or unsubstituted carbocyclic or heterocyclic ring structure.Such compounds are exemplified by, and include all the couplersdisclosed in GB 2018453A to Willis, hereby incorporated by reference inits entirety.

Such couplers have the property that they are relatively inactive ascouplers. This allows them to function as Dox scavengers to maximizeDmax in the positive image while, at the same time, minimizing the Dmin(or Dmax of the temporary or low-contrast negative image) during thermaldevelopment.

Some phenolic couplers may also behave as thermal solvents. It ispreferable that one material satisfy more than one function, but it isnot necessary.

Examples of phenolic couplers include:

As indicated above, a photothermographic process typically employsblocked developers that decompose (i.e., unblock) on thermal activationto release a developing agent. By a “dry thermal process” or “dryphotothermographic” process is meant herein a process involving, afterimagewise exposure of the photothermographic element, developing theresulting latent image by the use of heat to raise the temperature ofthe photothermographic element or film to a temperature of at leastabout 150° C., preferably at least about 155° C., more preferably atabout 160° C. to 180° C., without liquid processing of the film,preferably in an essentially dry process without the application ofaqueous solutions. By an essentially dry process is meant a process thatdoes not involve the uniform saturation of the film with a liquid,solvent, or aqueous solution. Thus, contrary to photothermographicprocessing involving low-volume liquid processing, the amount of waterrequired is less than 1 times, preferably less than 0.4 times and morepreferably less than 0.1 times the amount required for maximallyswelling total coated layers of the film excluding a back layer. Mostpreferably, no liquid is required or applied added to the film duringthermal treatment. Preferably, no laminates are required to beintimately contacted with the film in the presence of aqueous solution.

Preferably, during thermal development an internally located blockeddeveloping agent in reactive association with each of light-sensitivelayers becomes unblocked to form a developing agent, whereby theunblocked developing agent is imagewise oxidized on development and thisoxidized form reacts with the dye-providing couplers or other Doxscavenger.

The components of the photothermographic element can be in any locationin the element that provides the desired image. If desired, one or moreof the components can be in one or more layers of the element. Forexample, in some cases, it is desirable to include certain percentagesof the reducing agent, toner, thermal solvent, stabilizer and/or otheraddenda in the overcoat layer over the photothermographic imagerecording layer of the element. This, in some cases, reduces migrationof certain addenda in the layers of the element.

It is necessary that the components of the photographic combination be“in association” with each other in order to produce the desired image.The term “in association” herein means that in the photothermographicelement the photographic silver halide and other components of theimage-forming combination are in a location with respect to each otherthat enables the desired processing and forms a useful image. This mayinclude the location of components in different layers.

Preferably, development processing is carried out (i) for less than 60seconds, (ii) at the temperature from 150 to 200° C., and (iii) withoutthe application of any aqueous solution.

In view of advances in the art of scanning technologies, it has nowbecome natural and practical for photothermographic films such asdisclosed in EP 0762 201 to be scanned, which can be accomplishedwithout the necessity of removing the silver or silver halide from thenegative, although special arrangements for such scanning can be made toimprove its quality. See, for example, Simons U.S. Pat. No. 5,391,443.Method for the scanning of such films are also disclosed in commonlyassigned U.S. Pat. No. 6,521,384, issued Feb. 18, 2003, herebyincorporated by reference in its entirety.

A simple technique is to scan the photographic element point-by-pointalong a series of laterally offset parallel scan paths. A sensor thatconverts radiation received into an electrical signal notes theintensity of light passing through the element at a scanning point. Mostgenerally this electronic signal is further manipulated to form a usefulelectronic record of the image. For example, the electrical signal canbe passed through an analog-to-digital converter and sent to a digitalcomputer together with location information required for pixel (point)location within the image. The number of pixels collected in this mannercan be varied as dictated by the desired image quality. Very lowresolution images can have pixel counts of 192×128 pixels per filmframe, low resolution 384×256 pixels per frame, medium resolution768×512 pixels per frame, high resolution 1536×1024 pixels per frame andvery high resolution 3072×2048 pixels per frame or even 6144×4096 pixelsper frame or even more. Higher pixel counts or higher resolutiontranslates into higher quality images because it enables highersharpness and the ability to distinguish finer details especially athigher magnifications at viewing. These pixel counts relate to imageframes having an aspect ratio of 1.5 to 1. Other pixel counts and frameaspect ratios can be employed as known in the art. Most generally, adifference of four times between the number of pixels rendered per framecan lead to a noticeable difference in picture quality, whiledifferences of sixteen times or sixty four times are even more preferredin situations where a low quality image is to be presented for approvalor preview purposes but a higher quality image is desired for finaldelivery to a customer. On digitization, these scans can have a bitdepth of between 6 bits per color per pixel and 16 bits per color perpixel or even more. The bit depth can preferably be between 8 bits and12 bits per color per pixel. Larger bit depth translates into higherquality images because it enables superior tone and color quality.

Both large and small format frames are used in radiography. Mostradiography is done with monochrome films and the image is digitizedusing a 12 bit or 14 bit grey scale. Many different film sizes are usedfor medical diagnosis in humans. Intra-oral dental radiography forhumans uses frame sizes from 22 mm×35 mm to as large as 57 mm×76 mm.Medical imaging application for humans uses a variety of frame sizesdepending on the patient size with the largest common frame size beingaround 43 cm×43 cm. The resolution required for scanning of frames formedical applications depends on the type of exam performed. Mammographyexams using 24 cm×30 cm frame sizes can be digitized at 12 bits depthgrey scale and, for example, 3200×4600 pixels per frame. Chest examsgenerally use larger frames' (35 cm×43 cm) and often are digitized atlower resolution (for example, 2800×3400 pixels per frame) with a greyscale bit depth of 12. Veterinary applications can use the similar ordifferent film and frame sizes according to the application. Industrialradiography can use much large film sizes for the radiographicexaminations of, for example, structural defects in welds. Film sizes upto 40 cm×80 cm or larger can be used. Digitization of industrial filmsis done with varying frame size and resolution depending on the image.

The electronic signal can form an electronic record that is suitable toallow reconstruction of the image into viewable forms such as computermonitor displayed images, television images, optically, mechanically ordigitally printed images and displays and so forth all as known in theart. The formed image can be stored or transmitted to enable furthermanipulation or viewing, such as in U.S. Ser. No. 09/592,816 titled ANIMAGE PROCESSING AND MANIPULATION SYSTEM to Richard P. Szajewski, AlanSowinski and John Buhr.

The support for the photothermographic element can be either reflectiveor transparent, which is usually preferred. When reflective, the supportis white and can take the form of any conventional support currentlyemployed in print elements. When the support is transparent, it can becolorless or tinted and can take the form of any conventional supportcurrently employed in photographic film elements—e.g., a colorless ortinted transparent film support. Details of support construction arewell understood in the art. Examples of useful supports arepoly(vinylacetal) film, polystyrene film, poly(ethyleneterephthalate)film, poly(ethylene naphthalate) film, polycarbonate film, and relatedfilms and resinous materials, as well as paper, cloth, glass, metal, andother supports that withstand the anticipated processing conditions.

The element can contain additional layers, such as filter layers,interlayers, overcoat layers, subbing layers, antihalation layers andthe like. Transparent and reflective support constructions, includingsubbing layers to enhance adhesion, are disclosed in Section XV ofResearch Disclosure I.

Photographic elements of the present invention may also usefully includea magnetic recording material as described in Research Disclosure, Item34390, November 1992, or a transparent magnetic recording layer such asa layer containing magnetic particles on the underside of a transparentsupport as in U.S. Pat. No. 4,279,945, and U.S. Pat. No. 4,302,523.

Any convenient selection from among conventional radiation-sensitivesilver-halide emulsions can be incorporated within the layer units andused to provide the spectral absorptances of the invention. Mostcommonly, high bromide emulsions containing a minor amount of iodide areemployed. Radiation-sensitive silver chloride, silver bromide, silveriodobromide, silver iodochloride, silver chlorobromide, silverbromochloride, silver iodochlorobromide and silver iodobromochloridegrains are all contemplated. The grains can be either regular orirregular (e.g., tabular). Tabular grain emulsions, those in whichtabular grains account for at least 50 (preferably at least 70 andoptimally at least 90) percent of total grain projected area areparticularly advantageous for increasing speed in relation togranularity. To be considered tabular a grain requires two majorparallel faces with a ratio of its equivalent circular diameter (ECD) toits thickness of at least 2. Specifically preferred tabular grainemulsions are those having a tabular grain average aspect ratio of atleast 5 and, optimally, greater than 8. Preferred mean tabular grainthickness are less than 0.3 μm (most preferably less than 0.2 μm). Ultrathin tabular grain emulsions, those with mean tabular grain thickness ofless than 0.07 μm, are specifically contemplated. The grains preferablyform surface latent images so that they are capable of producingnegative images when processed in a solution surface developer.

Illustrations of conventional radiation-sensitive silver halideemulsions are provided by Research Disclosure I, cited above, I.Emulsion grains and their preparation. Chemical sensitization of theemulsions, which can take any conventional form, is illustrated insection IV. Chemical sensitization. Compounds useful as chemicalsensitizers, include, for example, active gelatin, sulfur, selenium,tellurium, gold, platinum, palladium, iridium, osmium, rhenium,phosphorous, or combinations thereof. Chemical sensitization isgenerally carried out at pAg levels of from 5 to 10, pH levels of from 4to 8, and temperatures of from 30 to 80° C. Spectral sensitization andsensitizing dyes, which can take any conventional form, are illustratedby section V. Spectral sensitization and desensitization. The dye may beadded to an emulsion of the silver halide grains and a hydrophiliccolloid at any time prior to (e.g., during or after chemicalsensitization) or simultaneous with the coating of the emulsion on aphotographic element. The dyes may, for example, be added as a solutionin water or an alcohol or as a dispersion of solid particles. Theemulsion layers also typically include one or more antifoggants orstabilizers, which can take any conventional form, as illustrated bysection VII. Antifoggants and stabilizers.

The silver-halide grains to be used in the invention may be preparedaccording to methods known in the art, such as those described inResearch Disclosure I, cited above, and T. H. James, The Theory of thePhotographic Process, Fourth Edition, Macmillan Publishing Co., Inc.,1977. These include methods such as ammoniacal emulsion making, neutralor acidic emulsion making, and others known in the art. These methodsgenerally involve mixing a water soluble silver salt with a watersoluble halide salt in the presence of a protective colloid, andcontrolling the temperature, pAg, pH values, etc, at suitable valuesduring formation of the silver halide by precipitation.

In the course of grain precipitation one or more dopants (grainocclusions other than silver and halide) can be introduced to modifygrain properties. For example, any of the various conventional dopantsdisclosed in Research Disclosure I, Section I. Emulsion grains and theirpreparation, sub-section G. Grain modifying conditions and adjustments,paragraphs (3), (4) and (5), can be present in the emulsions of theinvention. In addition it is specifically contemplated to dope thegrains with transition metal hexacoordination complexes containing oneor more organic ligands, as taught by Olm et al. U.S. Pat. No.5,360,712, the disclosure of which is here incorporated by reference.

It is specifically contemplated to incorporate in the face centeredcubic crystal lattice of the grains a dopant capable of increasingimaging speed by forming a shallow electron trap (hereinafter alsoreferred to as a SET) as discussed in Research Disclosure, Item 36736,November 1994, herein incorporated by reference.

The photographic elements of the present invention, as is typical,provide the silver halide in the form of an emulsion. Photographicemulsions generally include a vehicle for coating the emulsion as alayer of a photographic element. Useful vehicles include both naturallyoccurring substances such as proteins, protein derivatives, cellulosederivatives (e.g., cellulose esters), gelatin (e.g., alkali-treatedgelatin such as cattle bone or hide gelatin, or acid treated gelatinsuch as pigskin gelatin), deionized gelatin, gelatin derivatives (e.g.,acetylated gelatin, phthalated gelatin, and the like), and others asdescribed in Research Disclosure I. Also useful as vehicles or vehicleextenders are hydrophilic water-permeable colloids. These includesynthetic polymeric peptizers, carriers, and/or binders such aspoly(vinyl alcohol), poly(vinyl lactams), acrylamide polymers, polyvinylacetals, polymers of alkyl and sulfoalkyl acrylates and methacrylates,hydrolyzed polyvinyl acetates, polyamides, polyvinyl pyridine,methacrylamide copolymers. The vehicle can be present in the emulsion inany amount useful in photographic emulsions. The emulsion can alsoinclude any of the addenda known to be useful in photographic emulsions.

While any useful quantity of light sensitive silver, as silver halide,can be employed in the elements useful in this invention, it ispreferred that the total quantity be less than 10 g/m² of silver. Silverquantities of less than 7 g/m² are preferred, and silver quantities ofless than 5 g/m² are even more preferred. The lower quantities of silverimprove the optics of the elements, thus enabling the production ofsharper pictures using the elements. These lower quantities of silverare additionally important in that they enable rapid development anddesilvering of the elements.

The photographic elements may further contain other image-modifyingcompounds such as “Development-Inhibitor-Releasing” compounds (DIR's).Useful additional DIR's for elements of the present invention, are knownin the art and examples are described in U.S. Pat. Nos. 3,137,578;3,148,022; 3,148,062; 3,227,554; 3,384,657; 3,379,529; 3,615,506;3,617,291; 3,620,746; 3,701,783; 3,733,201; 4,049,455; 4,095,984;4,126,459; 4,149,886; 4,150,228; 4,211,562; 4,248,962; 4,259,437;4,362,878; 4,409,323; 4,477,563; 4,782,012; 4,962,018; 4,500,634;4,579,816; 4,607,004; 4,618,571; 4,678,739; 4,746,600; 4,746,601;4,791,049; 4,857,447; 4,865,959; 4,880,342; 4,886,736; 4,937,179;4,946,767; 4,948,716; 4,952,485; 4,956,269; 4,959,299; 4,966,835;4,985,336 as well as in patent publications GB 1,560,240; GB 2,007,662;GB 2,032,914; GB 2,099,167; DE 2,842,063, DE 2,937,127; DE 3,636,824; DE3,644,416 as well as the following European Patent Publications:272,573; 335,319; 336,411; 346,899; 362,870; 365,252; 365,346; 373,382;376,212; 377,463; 378,236; 384,670; 396,486; 401,612; 401,613. DIRcompounds are also disclosed in “Developer-Inhibitor-Releasing (DIR)Couplers for Color Photography,” C. R. Barr, J. R. Thirtle and P. W.Vittum in Photographic Science and Engineering, Vol. 13, p. 174 (1969),incorporated herein by reference.

It is common practice to coat one, two or three separate emulsion layerswithin a single image-forming layer unit. When two or more emulsionlayers are coated in a single layer unit, they are typically chosen todiffer in sensitivity. When a more sensitive emulsion is coated over aless sensitive emulsion, a higher speed is realized than when the twoemulsions are blended. When a less sensitive emulsion is coated over amore sensitive emulsion, a higher contrast is realized than when the twoemulsions are blended. It is preferred that the most sensitive emulsionbe located nearest the source of exposing radiation and the slowestemulsion be located nearest the support.

One or more of the layer units of the photothermographic embodiment ofthe invention is preferably subdivided into at least two, and morepreferably three or more sub-unit layers. It is preferred that alllight-sensitive silver-halide emulsions in the image recording unit havespectral sensitivity in the same region of the ultraviolet or visiblespectrum. In this embodiment, while all silver-halide emulsionsincorporated in the unit have spectral absorptances according toinvention, it is expected that there are minor differences in spectralabsorptance properties between them. In still more preferredembodiments, the sensitizations of the slower silver-halide emulsionsare specifically tailored to account for the light-shielding effects ofthe faster silver-halide emulsions of the layer unit that reside abovethem, in order to provide an imagewise uniform spectral response by thephotographic recording material as exposure varies with low to highlight levels. Thus higher proportions of peak light absorbing spectralsensitizing dyes may be desirable in the slower emulsions of thesubdivided layer unit to account for on-peak shielding and broadening ofthe underlying layer spectral sensitivity.

The photothermographic element may comprise an antihalation layer unitthat contains a decolorizable light absorbing material, such as one or acombination of pigments and dyes. Suitable materials can be selectedfrom among those disclosed in Research Disclosure I, Section VIII.Absorbing materials.

The photothermographic element may further comprise a surface overcoatSOC which are typically hydrophilic colloid layers that are provided forphysical protection of the elements during handling and processing. EachSOC also provides a convenient location for incorporation of addendathat are most effective at or near the surface of the element. In someinstances the surface overcoat is divided into a surface layer and aninterlayer, the latter functioning as spacer between the addenda in thesurface layer and the adjacent recording layer unit. In another commonvariant form, addenda are distributed between the surface layer and theinterlayer, with the latter containing addenda that are compatible withthe adjacent recording layer unit. Most typically the SOC containsaddenda, such as coating aids, plasticizers and lubricants, antistatsand matting agents, such as illustrated by Research Disclosure I,Section IX. Coating physical property modifying addenda. The SOCoverlying the emulsion layers optionally contains an ultravioletabsorber, such as illustrated by Research Disclosure I, Section VI. UVdyes/optical brighteners/luminescent dyes, paragraph (1).

Elements having excellent light sensitivity are best employed in thepractice of this invention. Photothermographic elements should have asensitivity of at least about ISO 1, preferably have a sensitivity of atleast about ISO 100, and more preferably have a sensitivity of at leastabout ISO 400. Elements having a sensitivity of up to ISO 20000 or evenhigher are specifically contemplated. The speed, or sensitivity, of aphotographic element is inversely related to the exposure required toenable the attainment of a specified density above Dmin afterprocessing.

Photographic speed for a reversal black-and-white film element has beenspecifically defined by the Federal Standard Relative Sensitivity,Method B (Fed. Std. No. 170a, Mar. 31, 1967) and relates specificallythe exposure H (in lux-seconds) at the point on the total density versuslog exposure curve where the density is 1.00 greater than base plusminimum density. Speed equals 10/H. Photographic speed and the speedclass system used to describe radiographic film and film coupled withintensifying elements such as film-intensifying screen systems isdiscussed by T. S. Curry, J. E. Dowdey, and R. C. Murray, Jr. inChristensen's Physics of Diagnostic Radiology 4^(th) edition (Lea andFebiger, Philadelphia, 1990, chapter 11). The photothermographic elementfor imaging ionizing radiation should have a speed class greater than50.

A photothermographic device, comprising photothermographic materials incombination with intensifying means, in accordance with the presentinvention, can be imagewise exposed to ionizing radiation using contactmethods by contacting the surface of the device, a cassette or otherassembly containing the photothermographic material to the surfaceemitting ionizing radiation such as is found in the technique of contactprinting of images produced by radioactive isotopes in anelectrophoresis gel where a film is placed directly in contact with theelectrophoresis gel for a certain amount of time for exposure afterwhich the film is developed. Exposures are monochromatic,orthochromatic, or panchromatic depending upon the spectralsensitization of the photographic silver halide.

The photothermographic elements of the present invention are preferablyof type B as disclosed in Research Disclosure I. Type B elements containin reactive association a photosensitive silver halide, a reducing agentor developer, optionally an activator, a coating vehicle or binder, anda salt or complex of an organic compound with silver ion. In thesesystems, this organic complex is reduced during development to yieldsilver metal, the organic silver salt is referred to as the silverdonor. References describing such imaging elements include, for example,U.S. Pat. Nos. 3,457,075; 4,459,350; 4,264,725; and 4,741,992. In thetype B photothermographic material it is believed that the latent imagesilver from the silver halide acts as a catalyst for the describedimage-forming combination upon processing. In these systems, a preferredconcentration of photographic silver halide is within the range of 0.01to 100 moles of photographic silver halide per mole of silver donor inthe photothermographic material.

The Type B photothermographic element comprises an oxidation-reductionimage forming combination that contains an organic silver salt oxidizingagent. The organic silver salt is a silver salt which is comparativelystable to light, but aids in the formation of a silver image when heatedto 80° C. or higher in the presence of an exposed photocatalyst (i.e.,the photosensitive silver halide) and a reducing agent.

The photosensitive silver-halide grains and the organic silver salts ofthe present invention can be coated so that they are in catalyticproximity during development. They can be coated in contiguous layers,but are preferably mixed prior to coating. Conventional mixingtechniques are illustrated by Research Disclosure, Item 17029 (June1978), as well as U.S. Pat. No. 3,700,458 and published Japanese patentapplications Nos. 32928/75, 13224/74, 17216/75, and 42729/76.

Examples of preferred blocked developers that can be used inphotographic elements of the present invention include, but are notlimited to, the blocked developing agents described in U.S. Pat. No.3,342,599, to Reeves; Research Disclosure (129 (1975) pp. 27-30)published by Kenneth Mason Publications, Ltd., Dudley Annex, 12a NorthStreet, Emsworth, Hampshire PO10 7DQ, ENGLAND; U.S. Pat. No. 4,157,915,to Hamaoka et al.; U.S. Pat. No. 4,060,418, to Waxman and Mourning; andin U.S. Pat. No. 5,019,492. Particularly useful are those blockeddevelopers described in U.S. Pat. Nos. 6,506,546; 6,306,551; 6,426,179;and 6,312,879. Further improvements in blocked developers are disclosedin U.S. Pat. Nos. 6,413,708; 6,543,226; 6,319,640; and 6,537,712. Yetother improvements in blocked developers and their use inphotothermographic elements are found in U.S. Pat. Nos. 6,506,528 and6,472,111.

In one embodiment of the invention blocked developer for use in thepresent invention may be represented by the following Structure V:DEV—(LINK 1)_(l)—(TIME)_(m)—(LINK 2)_(n)—B  Vwherein,

-   -   DEV is a silver halide color developing agent;    -   LINK 1 and LINK 2 are linking groups;    -   TIME is a timing group;    -   l is 0 or 1;    -   m is 0, 1, or 2;    -   n is 0 or 1;    -   l+n is 1 or 2;    -   B is a blocking group or B is:        —B′—(LINK 2)_(n)—(TIME)_(m)—(LINK 1)_(l)—DEV    -   wherein B′ also blocks a second developing agent DEV.

In a preferred embodiment of the invention, LINK 1 or LINK 2 are ofStructure VI:

wherein

-   -   X represents carbon or sulfur;    -   Y represents oxygen, sulfur of N—R₁, where R₁ is substituted or        unsubstituted alkyl or substituted or unsubstituted aryl;    -   p is 1 or 2;    -   Z represents carbon, oxygen or sulfur;    -   r is 0 or 1;        with the proviso that when X is carbon, both p and r are 1, when        X is sulfur, Y is oxygen, p is 2 and r is 0;    -   # denotes the bond to PUG (for LINK 1) or TIME (for LINK 2):    -   $ denotes the bond to TIME (for LINK 1) or T_((t)) substituted        carbon (for LINK 2).

Illustrative linking groups include, for example,

TIME is a timing group. Such groups are well-known in the art such as(1) groups utilizing an aromatic nucleophilic substitution reaction asdisclosed in U.S. Pat. No. 5,262,291; (2) groups utilizing the cleavagereaction of a hemiacetal (U.S. Pat. No. 4,146,396, Japanese Applications60-249148; 60 -249149); (3) groups utilizing an electron transferreaction along a conjugated system (U.S. Pat. Nos. 4,409,323; 4,421,845;Japanese Applications 57-188035; 58-98728; 58-209736; 58-209738); and(4) groups using an intramolecular nucleophilic substitution reaction(U.S. Pat. No. 4,248,962).

Other blocked developers that can be used are, for example, thoseblocked developers disclosed in U.S. Pat. No. 6,303,282 B1 to Naruse etal., U.S. Pat. No. 4,021,240 to Cerquone et al., U.S. Pat. No. 5,746,269to Ishikawa, U.S. Pat. No. 6,130,022 to Naruse, and U.S. Pat. No.6,177,227 to Nakagawa, and substituted derivatives of these blockeddevelopers. Although the present invention is not limited to any type ofdeveloping agent or blocked developing agent, the following are merelysome examples of some photographically useful blocked developers thatmay be used in the invention to produce developers during heatdevelopment.

In the preferred embodiment, the blocked developer is preferablyincorporated in one or more of the imaging layers of the imagingelement. The amount of blocked developer used is preferably 0.01 to 5g/m², more preferably 0.1 to 2 g/m² and most preferably 0.3 to 2 g/m² ineach layer to which it is added. These may be color forming or non-colorforming layers of the element.

After imagewise exposure of the imaging element, the blocked developeris activated during processing of the imaging element by the presence ofacid or base, by heating the imaging element during processing of theimaging element, and/or by placing the imaging element in contact with aseparate element, such as a laminate sheet, during processing. Thelaminate sheet optionally contains additional processing chemicals suchas those disclosed in Sections XIX and XX of Research Disclosure I. Suchchemicals include, for example, sulfites, hydroxylamine, hydroxamicacids and the like, antifoggants, such as alkali metal halides, nitrogencontaining heterocyclic compounds, and the like, sequestering agentssuch as an organic acids, and other additives such as buffering agents,sulfonated polystyrene, stain reducing agents, biocides, desilveringagents, stabilizers and the like.

A reducing agent in addition to, or instead of, the blocked developermay be included in the photothermographic element. The reducing agentfor the organic silver salt may be any material, preferably organicmaterial, that can reduce silver ion to metallic silver. Conventionalphotographic developers such as 3-pyrazolidinones, hydroquinones,p-aminophenols, p-phenylenediamines and catechol are useful, buthindered phenol reducing agents are preferred. The reducing agent ispreferably present in a concentration ranging from 1 to 25 percent ofthe photothermographic layer.

A wide range of reducing agents has been disclosed in dry silver systemsincluding amidoximes such as phenylamidoxime, 2-thienylamidoxime andp-phenoxy-phenylamidoxime, azines (e.g.,4-hydroxy-3,5-dimethoxybenzaldehydeazine); a combination of aliphaticcarboxylic acid aryl hydrazides and ascorbic acid, such as2,2′-bis(hydroxymethyl)propionylbetaphenyl hydrazide in combination withascorbic acid; an combination of polyhydroxybenzene and hydroxylamine, areductone and/or a hydrazine, e.g., a combination of hydroquinone andbis(ethoxyethyl)hydroxylamine, piperidinohexose reductone orformyl-4-methylphenylhydrazine, hydroxamic acids such asphenylhydroxamic acid, p-hydroxyphenyl-hydroxamic acid, ando-alaninehydroxamic acid; a combination of azines andsulfonamidophenols, e.g., phenothiazine and2,6-dichloro-4-benzenesulfonamidophenol; bis-naphthols as illustrated by2,2′-dihydroxyl-1-binaphthyl,6,6′-dibromo-2,2′-dihydroxy-1,1′-binaphthyl, andbis(2-hydroxy-1-naphthyl)methane; a combination of bis-o-naphthol and a1,3-dihydroxybenzene derivative, (e.g., 2,4-dihydroxybenzophenone or2,4-dihydroxyacetophenone); 5-pyrazolones such as3-methyl-1-phenyl-5-pyrazolone; reductones as illustrated bydimethylaminohexose reductone, anhydrodihydroaminohexose reductone, andanhydrodihydro-piperidone-hexose reductone; sulfamidophenol reducingagents such as 2,6-dichloro-4-benzene-sulfon-amido-phenol, andp-benzenesulfonamidophenol; 2-phenylindane-1,3-dione and the like;chromans such as 2,2-dimethyl-7-t-butyl-6 -hydroxychroman;1,4-dihydropyridines such as2,6-dimethoxy-3,5-dicarbethoxy-1,4-dihydropyridene; bisphenols, e.g.,bis(2-hydroxy-3-t-butyl-5-methylphenyl)-methane;2,2-bis(4-hydroxy-3-methylphenyl)-propane;4,4-ethylidene-bis(2-t-butyl-6-methylphenol); and2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane; ascorbic acid derivatives,e.g., 1-ascorbyl-palmitate, ascorbylstearate and unsaturated aldehydesand ketones, such as benzyl and diacetyl; pyrazolidin-3-ones; andcertain indane-1,3-diones.

An optimum concentration of organic reducing agent in thephotothermographic element varies depending upon such factors as theparticular photothermographic element, desired image, processingconditions, the particular organic silver salt and the particularoxidizing agent.

It is contemplated that the photothermographic element contains athermal solvent. Examples of thermal solvents, for example,salicylanilide, phthalimide, N-hydroxyphthalimide,N-potassium-phthalimide, succinimide, N-hydroxy-1,8-naphthalimide,phthalazine, 1-(2H)-phthalazinone, 2-acetylphthalazinone, benzanilide,and benzenesulfonamide. Prior-art thermal solvents are disclosed, forexample, in U.S. Pat. No. 6,013,420 to Windender. Examples of toningagents and toning agent combinations are described in, for example,Research Disclosure, June 1978, Item No. 17029 and U.S. Pat. No.4,123,282.

Post-processing image stabilizers and latent image keeping stabilizersare useful in the photothermographic element. Any of the stabilizersknown in the photothermographic art are useful for the describedphotothermographic element. Illustrative examples of useful stabilizersinclude photolytically active stabilizers and stabilizer precursors asdescribed in, for example, U.S. Pat. No. 4,459,350. Other examples ofuseful stabilizers include azole thioethers and blocked azolinethionestabilizer precursors and carbamoyl stabilizer precursors, such asdescribed in U.S. Pat. No. 3,877,940.

The photothermographic elements preferably contain various colloids andpolymers alone or in combination as vehicles and binders and in variouslayers. Useful materials are hydrophilic or hydrophobic. They aretransparent or translucent and include both naturally occurringsubstances, such as gelatin, gelatin derivatives, cellulose derivatives,polysaccharides, such as dextran, gum arabic and the like; and syntheticpolymeric substances, such as water-soluble polyvinyl compounds likepoly(vinylpyrrolidone) and acrylamide polymers. Other syntheticpolymeric compounds that are useful include dispersed vinyl compoundssuch as in latex form and particularly those that increase dimensionalstability of photographic elements. Effective polymers include waterinsoluble polymers of acrylates, such as alkylacrylates andmethacrylates, acrylic acid, sulfoacrylates, and those that havecross-linking sites. Preferred high molecular weight materials andresins include poly(vinyl butyral), cellulose acetate butyrate,poly(methylmethacrylate), poly(vinylpyrrolidone), ethyl cellulose,polystyrene, poly(vinylchloride), chlorinated rubbers, polyisobutylene,butadiene-styrene copolymers, copolymers of vinyl chloride and vinylacetate, copolymers of vinylidene chloride and vinyl acetate, poly(vinylalcohol) and polycarbonates. When coatings are made using organicsolvents, organic soluble resins may be coated by direct mixture intothe coating formulations. When coating from aqueous solution, any usefulorganic soluble materials may be incorporated as a latex or other fineparticle dispersion.

Photothermographic elements as described can contain addenda that areknown to aid in formation of a useful image. The photothermographicelement can contain development modifiers that function as speedincreasing compounds, sensitizing dyes, hardeners, anti-static agents,plasticizers and lubricants, coating aids, brighteners, absorbing andfilter dyes, such as described in Research Disclosure, December 1978,Item No. 17643 and Research Disclosure, June 1978, Item No. 17029.

The layers of the photothermographic element are coated on a support bycoating procedures known in the photographic art, including dip coating,air knife coating, curtain coating or extrusion coating using hoppers.If desired, two or more layers are coated simultaneously.

A photothermographic element as described preferably comprises a thermalstabilizer to help stabilize the photothermographic element prior toexposure and processing. Such a thermal stabilizer provides improvedstability of the photothermographic element during storage. Preferredthermal stabilizers are 2-bromo-2-arylsulfonylacetamides, such as2-bromo-2-p-tolysulfonylacetamide; 2-(tribromomethylsulfonyl)benzothiazole; and6-substituted-2,4-bis(tribromomethyl)-s-triazines, such as 6-methyl or6-phenyl-2,4-bis(tribromomethyl)-s-triazine.

Imagewise exposure is preferably for a time and intensity sufficient toproduce a developable latent image in the photothermographic element.

After imagewise exposure of the photothermographic element, theresulting latent image can be developed in a variety of ways. Thesimplest is by overall heating the element to thermal processingtemperature. Heating means known in the photothermographic arts areuseful for providing the desired processing temperature for the exposedphotothermographic element. The heating means is, for example, a simplehot plate, iron, roller, heated drum, microwave heating means, heatedair, vapor or the like. It is contemplated that the design of theprocessor for the photothermographic element be compatible to the designof the cassette, cartridge, or film packet used for storage and use ofthe element. Further, data stored on the film or cartridge may be usedto modify processing conditions or scanning of the element. Methods foraccomplishing these steps in the imaging system are disclosed incommonly assigned, co-pending U.S. Pat. Nos. 6,062,746 and 6,048,110which are incorporated herein by reference. The use of an apparatuswhereby the processor can be used to write information onto the element,information which can be used to adjust processing, scanning, and imagedisplay is also envisaged. This system is disclosed in U.S. Pat. No.6,278,510 which are incorporated herein by reference.

Thermal processing is preferably carried out under ambient conditions ofpressure and humidity. Conditions outside of normal atmospheric pressureand humidity may be used.

It is contemplated that imaging elements of this invention may bescanned prior to the removal of silver halide from the element. Theremaining silver halide yields a turbid coating, and it is found thatimproved scanned image quality for such a system can be obtained by theuse of scanners that employ diffuse illumination optics. Any techniqueknown in the art for producing diffuse illumination can be used.Preferred systems include reflective systems, that employ a diffusingcavity whose interior walls are specifically designed to produce a highdegree of diffuse reflection, and transmissive systems, where diffusionof a beam of specular light is accomplished by the use of an opticalelement placed in the beam that serves to scatter light. Such elementscan be either glass or plastic that either incorporate a component thatproduces the desired scattering, or have been given a surface treatmentto promote the desired scattering.

One of the challenges encountered in producing images from informationextracted by scanning is that the number of pixels of informationavailable for viewing is only a fraction of that available from acomparable classical photographic print. It is, therefore, even moreimportant in scan imaging to maximize the quality of the imageinformation available. Enhancing image sharpness and minimizing theimpact of aberrant pixel signals (i.e., noise) are common approaches toenhancing image quality. A conventional technique for minimizing theimpact of aberrant pixel signals is to adjust each pixel density readingto a weighted average value by factoring in readings from adjacentpixels, closer adjacent pixels being weighted more heavily.

The elements of the invention can have density calibration patchesderived from one or more patch areas on a portion of unexposedphotographic recording material that was subjected to referenceexposures, as described by Wheeler et al U.S. Pat. No. 5,649,260, Koengat al U.S. Pat. No. 5,563,717, and by Cosgrove et al U.S. Pat. No.5,644,647.

Illustrative systems of scan signal manipulation, including techniquesfor maximizing the quality of image records, are disclosed by Bayer U.S.Pat. No. 4,553,156; Urabe et al U.S. Pat. No. 4,591,923; Sasaki et alU.S. Pat. No. 4,631,578; Alkofer U.S. Pat. No. 4,654,722; Yamada et alU.S. Pat. No. 4,670,793; Klees U.S. Pat. Nos. 4,694,342 and 4,962,542;Powell U.S. Pat. No. 4,805,031; Mayne et al U.S. Pat. No. 4,829,370;Abdulwahab U.S. Pat. No. 4,839,721; Matsunawa et al U.S. Pat. Nos.4,841,361 and 4,937,662; Mizukoshi et al U.S. Pat. No. 4,891,713;Petilli U.S. Pat. No. 4,912,569; Sullivan et al U.S. Pat. Nos. 4,920,501and 5,070,413; Kimoto et al U.S. Pat. No. 4,929,979; Hirosawa et al U.S.Pat. No. 4,972,256; Kaplan U.S. Pat. No. 4,977,521; Sakai U.S. Pat. No.4,979,027; Ng U.S. Pat. No. 5,003,494; Katayama et al U.S. Pat. No.5,008,950; Kimura et al U.S. Pat. No. 5,065,255; Osamu et al U.S. Pat.No. 5,051,842; Lee et al U.S. Pat. No. 5,012,333; Bowers et al U.S. Pat.No. 5,107,346; Telle U.S. Pat. No. 5,105,266; MacDonald et al U.S. Pat.No. 5,105,469; and Kwon et al U.S. Pat. No. 5,081,692. Techniques forcolor balance adjustments during scanning are disclosed by Moore et alU.S. Pat. No. 5,049,984 and Davis U.S. Pat. No. 5,541,645.

The photothermographic materials of the invention can be employed incombination with an intensifying means, for example a photothermographicelement in combination with intensifying screen, that is imagewiseexposed to ionizing radiation in the form of x-radiation produced by amedical or dental x-ray generator or ionizing radiation produced by anindustrial x-ray source such as a Cobalt-60 or Iridium-192. The energyspectrum of the x-radiation is chosen according to the application to beserved. In industrial radiography, peak energy levels are often inexcess of 150 kV. In medical radiography, peak energy levels rarelyexceed 150 kV. Low energy x-radiation exposures for purposes of medicalexamination are less than 40 kV. Mammography, which is commonlypracticed at 28 kV is an example of low energy medical radiography.Dental radiography, commonly practiced at 60 to 90 kV is an example ofintermediate energy medical radiography.

As mentioned above, the intensifying means for ionizing radiationconverts ionizing radiation into a lower-energy form (for example,lower-energy actinic or non-actinic radiation) that can be used toexpose the photothermographic element for the purpose of forming alatent image in the photothermographic receptor layer. In oneembodiment, the intensifying means is a phosphor that emits visiblelight upon exposure to ionizing radiation. Another intensifying means isa metal foil that emits lower energy beta particles upon exposure toionizing radiation. Another intensifying means is a phosphor screenwherein phosphor particles or amorphous scintillator particles aredispersed in a polymeric binder solution then coated on a support toform a fluorescent layer that can be used to imagewise expose a lightsensitive receptor layer upon irradiation with ionizing radiation.Another intensifying means is an x-ray sensitive phosphor layer incombination with a photocathode that emits photoelectrons in response toexposure to ionizing radiation wherein the photoelectrons areaccelerated by an external applied field to bombard a second phosphorscreen where the visible light emission from the second phosphor screenis used for the purpose of exposing the inventive photothermographicelement to form a latent image therein. Accordingly, intensifying meanscan comprise one or many stages of energy conversion and amplificationin any manner which is suitable for the intended application.

A photothermographic assembly can contain a phosphor intensifying meansplaced in a light-tight package, for example an intra-oral dental filmpacket, wherein an intensifying element comprises an intensifying screencontaining a phosphor that emits light upon exposure to ionizingradiation. In another configuration, the photothermographic element canbe placed in a light-tight package containing an intensifying elementcomprised of a metal foil screen where the function of the metal foilscreen is to act both as an intensifying element by absorbing radiation(and releasing lower energy ionizing radiation such as beta particles)and to act as a means of reducing backscatter of radiation that can bedetrimental to image quality when the inventive element is imagewiseexposed to ionizing radiation. The metal-foil intensifying element canbe selected from any metal having an atomic number greater than 3,preferably having an atomic number between 48 and 83. A preferred metalfor use in the foil screen has atomic number 50. Another preferred metalfor use in the foil screen has atomic number 82. Another preferred metalfor use in the foil screen has atomic number 83. The foil can be anythickness suitable for the intended use and ranges from 1 micron to 1 cmin thickness.

Another useful configuration surrounds the inventive photothermographicelement (the imaging element) with a liner made of fabric, paper,plastic or other material that will not transmit visible light in alight-tight package optionally containing an outer metal foil in orderto minimize the possibility of fogging of the inventive element byinadvertent exposure to visible light. A preferred embodiment, forexample, for the use of the inventive element in dental radiography, isto enclose the inventive photothermographic element and at least onephosphor intensifying means, at least one liner, and at least one metalfoil in a light tight package, wherein the inventive element andphosphor intensifying element is surrounded with a liner, wherein oneface of the liner that is not in contact with the inventivephotothermographic element is in contact with a metal foil screen toreduce backscatter of ionizing radiation.

The light-tight package can be made from any material that is suitablefor the intended purpose including metals, metallized foils, metallizedplastics, plastics, cloth, paper, plastic impregnated paper, inorganiccomposite materials including ceramics, or extruded resins or plasticsof any type. A preferred light-tight package is made from polyethyleneresin containing titanium dioxide to impart light opacity to the resinsuch as is used in intra-oral dental packages.

Sources of ionizing radiation include any means known in the art.Various means for controlling the exposure of ionizing radiation includetimers controlling shutters, dose meters controlling shutters, directcontrol of power to a generator of ionizing radiation like an x-raytube, and placing or removing the photothermographic element withrespect to the source of ionizing radiation. After imagewise exposure toionizing radiation a photothermographic assembly can be developedaccording to known methods described in the art.

Organic and inorganic phosphors (and amorphous scintillators) areintensifying means because the light emitted therefrom upon exposure toionizing radiation is used for the purpose of forming a latent image inthe photosensitive photothermographic receptor layer. Phosphor particleintensifying elements used in fluorescent layers such as intensifyingscreen can have any conventional particle size range and distribution.It is generally appreciated that sharper images are realized withsmaller mean particle sizes, but light emission efficiency declines withdecreasing particles size. Thus the optimum mean particle size for agiven application is a reflection of the balance between imaging speedand image sharpness desired. Conventional phosphor particles size rangesand distributions are illustrated in the phosphor teachings cited below.

Phosphors are materials that emit infrared, visible, or ultravioletradiation upon excitation. An intrinsic phosphor is a material that isnaturally, (that is, intrinsically) phosphorescent. An “activated”phosphor is one composed of a basic material which may or may not be anintrinsic phosphor, to which one or more dopant(s) has beenintentionally added. These dopants “activate” the phosphor can cause itto emit infrared, visible, or ultraviolet radiation. For example, inGd₂O₂S:Tb, Ce, the Tb atoms (the emitting center and one of thedopants/activators) give rise to the optical emission of the phosphor.The addition dopant of Ce in the Gd₂O₂S lattice is called a coactivatorand improves the overall emission from the Th atoms in the Gd₂O₂S:Tb, Cephosphor. Some phosphors, such as BaFBr:Eu,K,Na, are known as storagephosphors, In these materials the dopants are involved in the storage aswell as the emission of radiation.

Any conventional or useful phosphor can be used, singly or in mixtures,in the practice of this invention. More specific details of usefulphosphors are provided as follows. For example, useful phosphors aredescribed in numerous references relating to fluorescent intensifyingscreens including but not limited to Research Disclosure Vol 184, August1979, Item 18431, Section IX, X-ray screens/phosphors, and U.S. Pat. No.2,303,942 (Wynd et al.), U.S. Pat. No. 3,778,615 (Luckey), U.S. Pat. No.4,032,471 (Luckey), U.S. Pat. No. 4,225,653 (Brixner et al), U.S. Pat.No. 3,418,246 (Royce), U.S. Pat. No. 3,617,743 (Rabatin), U.S. Pat. No.3,974,389 (Ferri et al), U.S. Pat. No. 3,591,516 (Rabatin), U.S. Pat.No. 3,607,770 (Rabatin), U.S. Pat. No. 3,666,676 (Rabatin), U.S. Pat.No. 3,795,814 (Rabatin), U.S. Pat. No. 4,405,691 (Yale), U.S. Pat. No.4,311,487 (Luckey et al), U.S. Pat. No. 4,387,141 (Patten), U.S. Pat.No. 5,064,729 (Zegarski), U.S. Pat. No. 5,250,366 (Nakajima et al), thedisclosures of all of which are incorporated herein by reference withrespect to the phosphors.

Useful classes of phosphors include but are not limited to calciumtungstate, (CaWO₄), activated or unactived lithium stannates such asLi₂SnO₃:Ti, niobium and/or rare earth activated or unactivated yttrium,lutetium, or gadolinium tantalates, rare earth (such as terbium,lanthanum, gadolinium, cerium, and lutetium)-activated or unactivatedmiddle chalcogen phosphors such as rare earth oxychalcogenides andoxyhalides, and terbium activated or unactivated lanthanum and lutetiummiddle chalcogen phosphors, Still other useful phosphors are thosecontaining hafnium as described for example in U.S. Pat. No. 4,988,880(Bryan et al), U.S. Pat. No. 4,994,205 (Bryan et al), U.S. Pat. No.4,988,881 (Bryan et al), U.S. Pat. No. 5,095,218 (Bryan et al), U.S.Pat. No. 5,112,700 (Lambert et al), U.S. Pat. No. 5,124,072 (Dole et al)and U.S. Pat. No. 5,336,893 (Smith et al.) the disclosures of which areall incorporated herein by reference.

Preferred rare earth oxychalcogenide and oxyhalide phosphors arerepresented by the following formula:M′_((w-n))M″_(n)O_(w)X′wherein M′ is at least one of the metals yttrium (Y), lanthanum (La),gadolinium (Gd), or lutetium (Lu), M″ is bismuth (Bi) or at least one ofthe rare earth metals, preferable dysprosium (Dy), erbium (Er), europium(Eu), holmium (Ho), neodymium (Nd) praseodymium (Pr) samarium (Sm),tantalum (Ta) terbium (Th), thulium (Tm), or ytterbium (Yb), X′ is amiddle chalcogen. (S,Se,Te) or halogen, N is 0.002 to 0.2, and w is 1when X′ is a halogen or 2 when X′ is a middle chalcogen. These includerare earth activated lanthanum oxybromides and terbium activated orthulium activated gadolinium oxy sulfides such as Gd₂O₂S:Tb.

Other suitable phosphors are described in U.S. Pat. No. 4,835,397(Arakawa et al.) and U.S. Pat. No. 5,381,015 (Dooms), both incorporatedherein by reference, including for example, divalent europium and otherrare earth activated alkaline earth metal halide phosphors and rareearth element activated rare-earth oxyhalide phosphors. Of these typesof phosphors, the more preferred phosphors include alkaline earth metalfluorohalide prompt emitting and/or storage phosphors, particularlythose containing iodide such as alkaline earth metal fluorobromoiodidestorage phosphors as described in U.S. Pat. No. 5,464,568 (Bringley etal.), hereby incorporated herein by reference.

Another class of phosphors are those that include a rare earth host andare rare-earth activated mixed alkaline-earth metal sulfates such aseuropium-activated barium strontium sulfate.

Particularly useful phosphors are the doped or undoped tantalates suchas YTaO₄, YTaO₄:Nb, Y(Sr)TaO₄, Y(Sr)TaO₄:Nb, Mg₄Ta₂O₉ and Mg₄Ta₂O₉:Nb.These phosphors are described in U.S. Pat. No. 4,226,653 (Brixner), U.S.Pat. No. 5,064,729 (Zegarski), U.S. Pat. No. 5,250,366 (Nakajima etal.), U.S. Pat. No. 5,626,957 (Benso et al.) and U.S. Pat. No. 5,132,192(Sieber et al.) all incorporated herein by reference.

Still other useful phosphors are phosphate-containing phosphors based onthe monazite host lattice LaPO₄ corresponding to the formulaLa_((1-x-y))M′_(x)M″_(y)PO_(4-z)F_(z)wherein M′ may be one or a mixture of rare earth metal cation and M″ maybe one or a mixture of cations selected from Mg, Ca, Sr, Ba; 0<x<1 and0<y<0.5 and 0<z<0.5. This class of compounds includes the low cost greenemitting phosphor LaPO₄:Tb,Ce as well as the ultraviolet emittingphosphors LaPO₄:Ce and LaPO_(4-x) F_(x):Ce,Sr.

Other useful phosphors are alkaline-earth metal phosphors that can bethe products of firing starting materials comprising optional oxide anda combination of species characterized by the following formula:MFX_(1-z)I_(z)uM^(a)X^(a):yA:eQ:tDwherein “M” is magnesium (Mg), calcium (Ca), strontium (Sr) or barium(Ba); “F” is fluoride; “X” is chloride (Cl) or bromide (Br); “I” isiodide; M^(a) is sodium (Na), potassium (K), rubidium (Rb) or cesium(Cs); X^(a) is fluoride (F), chloride (Cl), bromide (Br) or iodide (1);“A” is europium (Eu), cerium (Ce), samarium (Sm) or terbium (Tb); and“Q” is BeO, MgO, CaO, SrO, BaO, ZnO, Al₂O₃, La₂O₃, In₂O₃, SiO₂, TiO₂,ZrO₂, GeO₂, SnO₂, Nb₂O₅, Ta₂O₅, or ThO₂; “D” is vanadium (V), chromium(Cr), manganese (Mn), iron (Fe), cobalt (Co), or nickel (Ni). Thenumbers in the noted formula are the following: “z” is 0 to 1, “u” isfrom 0 to 1; “y” is from 1×10⁻⁴ to 0.1; “e” is from 0 to 1; and “t” isfrom 0 to 0.01. These definitions apply wherever they are found in thisapplication unless specifically stated to the contrary. It is alsocontemplated that “M”, “X”, “A”, and “D” represent multiple elements inthe groups identified above.

Storage phosphors are also intensifying elements and can also be used inthe practice of this invention. Various storage phosphors are describedfor example in U.S. Pat. No. 5,464,568 (noted above) incorporated hereinby reference. Such phosphors include divalent alkaline-earth metalfluorohalide phosphors that may contain iodide and are the product offiring an intermediate, comprising oxide and a combination of speciescharacterized by the following formula:(Ba_(1-a-b-c)Mg_(a)Ca_(b)Sr_(c))FX_(1-z)I_(zr)M^(a)X^(a):yAwherein X, M^(a), X^(a), A, z and y have the same meanings as for thepreceeding formula, and the sum of a, b, and c is from 0 to 1, and r isfrom 1×10⁻⁶ to 0.1. Some embodiments of these phosphors are described inmore detail in U.S. Pat. No. 5,464,568 (noted above).

Still other storage phosphors are described in U.S. Pat. No. 4,368,390(Takahashi et al.) incorporated herein by reference and include divalenteuropium and other rare-earth activated alkaline-earth metal halides andrare-earth element activated rare-earth oxyhalides as described in moredetail above.

Still other examples of useful phosphors include SrS:Ce,Sm; SrS:Eu,Sm;ThO₂Er; La₂O₂S:Eu,Sm; ZnS:Cu,Pb; and others described in U.S. Pat. No.5,227,253 (Takasu et al.), incorporated herein by reference.

Still another useful class of phosphor intensifying elements for use inthe present invention are the activated alkali halide phosphors withmonovalent and divalent activator cations. Many alkali halide phosphorsare photostimulable storage phosphors as well as efficientthermoluminescent phosphors. Thermoluminscent phosphor intensifyingelements are phosphors that store the x-ray energy upon exposure andliberate this energy at a later time in the form of photons upon theapplication of heat. Examples of a storage phosphor that is alsothermoluminscent is CsBr:Eu. The properties of CsBr:Eu as a storagephosphor are well documented in the literature. (For example, see P.Hackenschmeied, G. Schierning, M. Batentschuk, and A. Winnacker inJournal of Applied Physics, 93(9), 5109, (2003)). The properties ofCsBr:Eu as an x-ray sensitive thermoluminescent material withthermoluminescent emission temperature at or below the processingtemperatures for the inventive photothermographic element described hereare described by Y. V. Zorenko, R. M. Turchak, and I. V. Konstankevychin Functional Materials, 10(1), 75, (2003).

The one or more phosphors used in the practice of this invention arepresent in the present photothermographic materials in an amount of atleast 0.01 mole per mole and preferably from about 0.1 to about 20 moleper mole of total silver in the photothermographic material.

It is advantageous to match the emission from the phosphor intensifyingmeans to the spectral sensitivity of the inventive photothermographicelement. Matching may involve either the use of appropriately sensitizedphotothermographic materials or the matching of the emission from theintensifying means to the native sensitivity of the photothermographicmaterials.

Because of the size of the phosphors used in the invention, generallythe layer in which they are incorporated (usually one or more emulsionlayers) have a dry coating weight of at least 5 g/m² and preferably fromabout 5 g/m² to about 200 g/m². In one configuration of the inventionthe one or more phosphors and the photosensitive silver halide areincorporated within the same imaging layer that has a dry coating weightwithin the noted preferred range.

In another configuration of the invention, the phosphor is in a separatelayer. Phosphors coated in a separate layer on a support are oftencalled x-ray intensifying screens. The support can be any suitablesubstrate known in the art. For example, a particularly useful supportis a polymeric support that is preferably a flexible, transparent oropaque film that has any desired thickness and is composed of one ormore polymeric materials. The support is required to exhibit dimensionalstability and to have suitable adhesive properties with overlyinglayers. Useful polymeric materials for making such supports includepolyesters (such as polyethylene terephthalate and polyethylenenaphthalate), cellulose acetate and other cellulose esters, polyvinylacetal, polyolefins, polycarbonates, and polystyrenes. Preferredsupports are composed of polymers having good heat stability, such aspolyesters and polycarbonates. Support materials may also be treated orannealed to reduce shrinkage and promote dimensional stability.

It is also useful to use supports comprising dichroic mirror layers asdescribed in U.S. Pat. No. 5,795,708 (Boutet), incorporated herein byreference.

Also useful are transparent, multilayer, polymeric supports comprisingnumerous alternating layers of at least two different polymericmaterials as described in U.S. Pat. No. 6,630,283 (Simpson et al.),incorporated herein by reference.

Opaque supports can also be used, such as dyed polymeric films andresin-coated papers that are dimensionally stable at room temperatureand at high temperatures.

Support materials can contain various colorants, pigments, antihalationor acutance dyes if desired. For example, the support can include one ormore dyes that provide a blue color in the resulting imaged film.Support materials may be treated using conventional procedures (such ascorona discharge) to improve adhesion of overlying layers, or subbing orother adhesion-promoting layers can be used.

The x-ray intensifying screens used in this invention are eitherseparate or integral with the inventive photothermographic element. Theycan be incorporated in a separate layer coated in a multiplayerphotothermographic

One configuration of an intensifying means is an intensifying screenwhich comprises, for example coated on a conventional support, a binderand a phosphor intensifying element. Sufficient binder to givestructural coherence to the layer is used. Binders for the phosphorlayers of intensifying screen are often selected for their wearresistance since screens are normally reused until physically worn. Inaddition, the binders employed in the intensifying screens and generallychosen from organic polymers which are transparent to x-radiation andthe light emitted from the phosphors. Various polymers used forintensifying screens include sodium o-sulfobenzaldehyde acetal ofpoly(vinyl alcohol); chlorosulfonated poly(ethylene); a mixture ofmacromolecular bisphenol poly(carbonates) and copolymers comprisingbisphenol carbonates and poly(alkylene oxides); aqueous ethanol solublenylons; poly(alkyl acrylates and meth-acrylates) and copolymers of alkylacrylates and methacrylates with acrylic and methacrylic acid;poly(vinyl butyal)s; and poly(urethane) elastomers. These and otheruseful binders are disclosed in U.S. Pat. Nos. 2,502,529; 2,887,379;3,617,285; 3,300,310; 3,300,311; and 3,743,833; and in ResearchDisclosure Vol 154, February 1977, Item 15444, and Vol. 182, June 1979.Particularly preferred intensifying screen binders are poly(urethanes)such as those commercially available under the trademark Estane fromGoodrich Chemical Co. the trademark Permuthane from the PermuthaneDivision of ICI, Ltd, and the Trademark Cargill from Cargill, Inc.

Any conventional ratio of phosphor to binder can be employed. Generallythinned fluorescent layer intensifying elements and sharper images arerealized when a high weight ratio of phosphor to binder is employed.Typical weight ratios of phosphor to binder are between 5:1 and 50:1 andmore preferably 10:1 to 25:1. In those instances in which it is desiredto improve the imaging performance of an intensifying screen withoutchanging its thickness, the fluorescent layer is modified to impart asmall but significant degree of light absorption within the screen. Ifthe binder is chosen to exhibit the desired degree of light absorptionthen no other ingredient of the fluorescent layer is required to performthe light attenuation function. For example, a slightly yellowtransparent polymer will absorb a significant fraction of phosphoremitted blue light. Ultraviolet absorption can be similarly achieved. Itis specifically noted that the less structurally complex chromophoresfor ultraviolet absorption particularly lend themselves to incorporationin binder polymers.

Where a separate absorber is incorporated in the phosphor layerintensifying element to improve the imaging performance of the layer,the absorber can be a dye or pigment capable of absorbing light withinthe spectrum emitted by the phosphor. Yellow dye or pigment selectivelyabsorbs blue light emissions and is particularly useful with a blueemitting phosphor. On the other hand, a green emitting phosphor isbetter used in combination with magenta dyes or pigments. Ultravioletemitting phosphors can be used with known ultraviolet absorbers. Blackdyes and pigments are, of course, generally useful with phosphorsbecause of their broad absorption spectra. Carbon black is a preferredlight absorber for incorporation in the fluorescent layers because ofits low cost and broad spectrum of absorption. Luckey and Cleare U.S.Pat. No. 4,259,588 here incorporated by reference, teaches thatincreased sharpness can be achieved by incorporating a yellow dye in aterbium activated gadolinium oxysulfide fluorescent layer.

The fluorescent layer intensifying element can, if desired, beconstructed of multiple fluorescent layers comprised of similar ordissimilar phosphors, however, it is preferred that the fluorescentlayer unit be constructed of a single fluorescent layer containing asingle phosphor.

Another configuration of the present invention involves coating thephotothermographic element of the invention directly onto the surface ofan intensifying screen intensifying element to form a unitaryphotothermographic imaging element according to the method described inU.S. Pat. No. 4,865,944, the disclosure of which is here incorporated byreference. In this configuration, the wear-resistant screen binders canbe used in the photothermographic element of this invention whenemployed in combination with subbing layers to achieve adhesion to thefilm support and novel interlayers to effect adhesion of the fluorescentlayer to the hydrophilic colloid binder of the photothermographicemulsion layer. It has been recognized that the types of polymersemployed to promote adhesion between gelatino-silver halide emulsionlayers and polyester film supports form generally satisfactoryfluorescent layer binders. In other words, the preferred binders for thefluorescent layers of the elements of this invention are the samebinders employed to form subbing layers on polyester film supports, suchas poly(ethylene terephthalate) film supports.

The following examples illustrate the practice of this invention. Theyare not intended to be exhaustive of all possible variations of theinvention. Parts and percentages are by weight unless otherwiseindicated.

EXAMPLES

Silver Salt Dispersion SS-1:

A stirred reaction vessel was charged with 480 g of lime processedgelatin and 5.6 l of distilled water. A solution containing 0.7 M silvernitrate was prepared (Solution A). A solution containing 0.7 Mbenzotriazole and 0.7 M NaOH was prepared (Solution B). The mixture inthe reaction vessel was adjusted to a pAg of 7.25 and a pH of 8.00 byadditions of Solution B, nitric acid, and sodium hydroxide as needed.

Solution A was added with vigorous mixing to the kettle at 38 cc/minute,and the pAg was maintained at 7.25 by a simultaneous addition ofsolution B. This process was continued until the quantity of silvernitrate added to the vessel was 3.54 M, at which point the flows werestopped and the mixture was concentrated by ultrafiltration. Theresulting silver salt dispersion contained fine particles of silverbenzotriazole.

Silver Salt Dispersion SS-2:

A stirred reaction vessel was charged with 480 g of lime processedgelatin and 5.6 l of distilled water. A solution containing 0.7 M silvernitrate was prepared (Solution A). A solution containing 0.7 M1-phenyl-5-mercaptotetrazole and 0.7 M NaOH was also prepared (SolutionB). The mixture in the reaction vessel was adjusted to a pAg of 7.25 anda pH of 8.00 by additions of Solution B, nitric acid, and sodiumhydroxide as needed.

Solution A was added to the kettle at 19.6 cc/minute, and the pAg wasmaintained at 7.25 by a simultaneous addition of solution B. Thisprocess was continued until the 3.54 moles of silver nitrate had beenadded to the vessel, at which point the flows were stopped and mixturewas concentrated by ultrafiltration. The resulting silver saltdispersion contained fine particles of the silver salt of1-phenyl-5-mercaptotetrazole.

Silver Salt Dispersion SS-3:

A silver salt dispersion comprising equal 2:1:1 molar ratio of silver:benzotriazole: 1-phenyl-5-mercaptotetrazole was prepared according tothe methods described for the individual silver salts of benzotriazole,and 1-phenyl-5-mercaptotetrazole.

Emulsion E-1:

Emulsion example E-1 is a bromoiodide emulsion containing tabular grainshaving a mean equivalent circular diameter of 0.3 μm and a meanthickness of 0.12 μm. The overall bulk iodide content was 4.5 mole %.

The emulsion was then chemically and spectrally sensitized to greenlight using the following spectral sensitizing dyes:

Spectral Sensitizing Dyes:

-   -   GSD-1:Anhydro-5-chloro-9        ethyl-5′-phenyl-3′-(3-sulfobutyl)-3-(3-sulfopropyl)-oxacarbocyanine        hydroxide, sodium salt.    -   GSD-5:        Anhydro-3,9-diethyl-3′-[N-(methylsulfonyl)carbamoylmethyl]-5-phenylbenzothiazolo        oxacarbocyanine hydroxide.

The chemical and spectral finish was in accordance with standard tradepractice for color negative film applications. When exposed to light,the silver halide grains form surface latent image that amplifies duringsolution development to form a silver/dye negative image. C-41 is atypical process.

Preparation of Silver Bromoiodide Emulsion E-2).

Emulsion E-2 is a silver bromoiodide emulsion containing tabular grainshaving a mean equivalent circular diameter of 0.6 μm. The emulsion wasoptimally chemically sensitized with sulfur and gold and spectrallypan-sensitized using known methods in the art with sensitizing dyesGSD-2, GSD-3 and GSD-4 in the relative amounts listed in Table 1. TABLE1

GSD-2 (0.138 g/mol silver)

GSD-3 (0.04 g/mol silver)

GSD-4 (0.231 g/mol silver)Developer Dispersion, DD-1:

A dispersion of developer D-17 was prepared by the method of ballmilling. For each gram of incorporated developer, 0.2 g of sodiumtri-isopropylnaphthalene sulfonate, 10 g of water, and 25 ml of beadswere added. Following milling, the zirconia beads were removed byfiltration. The slurry was refrigerated prior to use.

Thermal Solvent Dispersion, TSD-1:

A dispersion of salicylanilide (TS-1) was prepared by the method of ballmilling. A total of 19 g of slurry was produced by combining 3.0 gm TS-1solid, 0.20 g polyvinyl pyrrolidone, 0.20 g TRITON X-200 surfactant, and15.6 g distilled water. To this mixture was added 20 ml of zirconiabeads. The slurry was ball milled for 48 hours. Following milling, thezirconia beads were removed by filtration. At this point, 1 g of gelatinwas added, allowed to swell, and then dissolved in the mixture byheating at 40 C. The resulting mixture was chill set to yield adispersion containing 5% gelatin and 15% TS-1.

Phenolic Coupler Dispersion, PCD-1:

A dispersion of cyan coupler PC-1 was prepared by the method of ballmilling. A total of 200 g of slurry was produced by combining 20 g PC-1solid, 30 g of 10% oleylmethyltaurate, and 150 g distilled water. Tothis mixture was added 475 ml of 1.8 mm zirconia beads. The slurry wasball milled for 72 hours. Following milling, the zirconia beads wereremoved by filtration.

Phenolic Coupler Dispersion, PCD-2:

Phenolic coupler PC-1 (30 g) was dissolved in 60 g ethyl acetate at 60C. Another solution was prepared by combining 40 g gelatin, 337.5 gwater and 32.5 g of 10% 2-Naphthalenesulfonic acid,tris(1-methylethyl)-, sodium salt and heating at 50 C. The solutionswere combined and passed through a colloid mill five times. The ethylacetate was removed by rotary evaporation for 20 minutes.

Phenolic Coupler Dispersion, PCD-3:

A dispersion of phenolic coupler PC-2 was prepared by the method of ballmilling. A slurry was produced by combining 20 g PC-2 solid, 20 g of1,0% polyvinyl pyrrolidone, and 160.0 g distilled water. To this mixturewas added 475 ml of 1.8 mm zirconia beads. The slurry was ball milledfor 72 hours. Following milling, the zirconia beads were removed byfiltration.

Phenolic Coupler Dispersion, PCD-4:

A dispersion of phenolic coupler PC-3 was prepared by the method of ballmilling. A slurry was produced by combining 20 g PC-3 solid, 20 g of 10%polyvinyl pyrrolidone, and 160.0 g distilled water. To this mixture wasadded 475 ml of 1.8 mm zirconia beads. The slurry was ball milled for 72hours. Following milling, the zirconia beads were removed by filtration.

Phenolic Coupler Dispersion PCD-5:

A dispersion of catechol PC-4 was prepared by the method of ballmilling. A slurry was produced by combining 20 g PC-4 solid, 17.5 g of10% polyvinyl pyrrolidone, 2.5 g of 9.14% Pionin A44SP, and 162.5 gdistilled water. To this mixture was added 475 ml of 1.8 mm zirconiabeads. The slurry was ball milled for 72 hours. Following milling, thezirconia beads were removed by filtration.

Preparation of Intensifying Screen Intensifying Elements

Handcoatings of intensifying screen intensifying elements for ionizingradiation were prepared by dispersing commercially obtained phosphors ina 12% by weight solution of BUTVAR-79 in cyclohexanone. BUTVAR-79 isobtained from Wacker Chemical. Ultraviolet-emitting cerium activatedlanthanum phosphate (type NP-806) phosphor was obtained from NichiaCorporation of America. Red-emitting europium activated gadoliniumoxysulfide (type 3011-13) phosphor was obtained from Nichia Corporationof America. Green-emitting terbium activated lanthanum phosphatephosphor (type 2212) was obtained from Osram Sylvania. Blue-emittingeuropium activated barium strontium sulfate phosphor was obtained fromthe Health Imaging Division of Eastman Kodak Company. Blue-emittingXomatic Regular™ intensifying screens containing the europium activatedbarium strontium sulfate phosphor were obtained from the Health ImagingDivision of Eastman Kodak Company. Blue-emitting Hi Plus™ intensifyingscreens containing the intrinsic blue emitting phosphor calciumtungstate were obtained from E.I. Dupont De Nemours Company. Phosphordispersions for coating were prepared by adding 3.937 grams of phosphorto 10 mls of 12% BUTVAR-79 in cyclohexanone. The dispersions was rolledfor at least 24 hours before coating on an unsubbed 0.018 cm (0.007inches) thick poly(ethylene terephthalate) support using a blade with aclearance of 0.015 cm (0.006 inch) above the support to give a coatingwith approximate coating weight of phosphor equal to 60 grams per squaremeter. The coating was dried in air then further dried for 8 hours at 50degrees C. Intensifying screens with the phosphors LaPO₄:Ce, Gd₂O₂S:Euand LaPO₄:Tb,Ce were prepared as described above. The dried screens werethen cut to the desired size as appropriate for the examples describedbelow.

Example 1

The following aqueous multilayer coating, in Table 2, was prepared usingnegative-working emulsion E-2 according to methods known in the art. Thesupport was 0.018 cm (0.007 inch) thick poly(ethylene terephthalate).TABLE 2 Component g/m² Layer 1: Interlayer Gelatin Ethene, 1,1′- 0.14(methylenebis(sulfonyl))bis- Layer 2: Imaging Layer Pan-Sensitive Silver(from .54 emulsion E-2) Silver (from silver salt SS-1) 1.08 Silver (fromsilver salt SS-2) 1.08 Phenolic Coupler PC-4 (from 1.08 PCD-5) DeveloperD-17 (from DD-1) 1.08 Salicylanilide (from TSD-1) 2.16 Gelatin 6.88Layer 3: Overcoat Gelatin 3.23 Surfactant SF-1 0.01 Ludox ® AM(colloidal silica) 0.15

Example 2

Example 2 illustrates the sensitivity of coating Example 1 when combinedwith several phosphor intensifying screens. Coating Example 1 was placedin face-side contact with each screen and the combined film packet wasexposed to x-ray radiation with the backside of the intensifying screenfacing the x-ray source. The x-ray exposures ranged from 0.09 mRems to128.8 mRems. Table 3 below lists the peak wavelength emitted by eachscreen, Table 4 shows the x-ray source settings for each exposure level,and Table 5 lists, for each exposure level, the developed bluetransmission density after thermally processing coating Example 1 at162° C. for 30 sec. FIG. 1 shows the H & D photographic curve (developedblue transmission density vs relative log exposure level) which has adecreasing slope indicating a positive response (decreasing density withincreasing exposure). TABLE 3 Phosphor ID 1 2 3 4 5 Description LaPO₄:CeXOMATIC Dupont LaPO₄: Gd₂O₂S: REG HI PLUS Ce, Tb Eu Coverage 60g/m{circumflex over ( )}2 60 60 g/m{circumflex over ( )}2 g/m{circumflexover ( )}2 Commercial Nichia Osram Nichia Source NP-806 2212 3011-13peak 318 390 450 544 626 emission (nm)

TABLE 4 Relative Theor. Measured Distance Cu* Al LUCITE exposure mRemsmRems mA mSec KVp (cm) mAs log(E) mirror (mm) (mm) (cm) 0.5 1 0.09 10050 57 177.8 5 −1.045757 no 0.5 1 5.08 1 2 1.02 200 200 60 177.8 400.0086 no 0.5 1 5.08 4 8 4.01 510 300 60 177.8 153 0.603144 no 0.5 15.08 16 32 15.98 570 300 60 97.8 171 1.203577 no 0.5 1 5.08 32 64 31.9570 600 60 97.8 342 1.503791 no 0.5 1 5.08 64 128 63.9 570 1200 60 97.8684 1.805501 no 0.5 1 5.08 128 256 128.8 110 140 60 97.8 15.4 2.109916no 0 0 0

TABLE 5 Measured Blue density by Phosphor ID MRems log(E) 1 2 3 4 5 0.09−1.05 3.78 3.627 3.611 3.603 3.576 1.02 0.01 3.46 3.312 3.352 3.4573.286 4.01 0.60 3.498 2.009 1.924 3.406 3.376 15.98 1.20 3.593 0.680.676 3.386 3.287 31.9 1.50 3.422 0.657 0.659 3.139 2.959 63.9 1.813.331 0.688 0.681 2.555 2.22 128.8 2.11 3.106 0.694 0.7 1.774 0.98

The above results show that phosphor screens 2 and 3 produced thefastest photographic response in combination with coating Example 1,because of the relatively higher phosphor coverage of these commercialphosphor screens (intensifying elements). The exposure necessary toachieve a minimum blue density is approximately 16 mRem (correspondingto 1.20 relative log exposure in FIG. 1). The unexposed areas of coatingExample 1 thermally developed to a blue transmission density exceeding3.5.

Example 3

The following aqueous single layer coating shown in Table 6 was preparedusing negative working emulsion E-1 and silver salt SS-3. The supportwas 0.018 cm (0.007 inch) thick poly(ethylene terephthalate). TABLE 6Component g/m² Layer 1: Imaging Layer Green Sensitive Silver (fromemulsion E-1) 0.46 Silver (from silver salt SS-3) 0.46 Phenolic CouplerPC-4 (from PCD-3) 1.12 Developer D-17 (from DD-1) 0.34 Salicylanilide(from TSD-1) 0.86 Gelatin 3.77

Example 4

Single layer coating Examples 4 incorporates a BaSO₄.Sr,Eu phosphordirectly in the photothermographic element. The preparation followscoating Example 3 except for the addition of blue light emittingphosphor BaSO₄.Sr,Eu (emits at 390 nm and is the same phosphor inXOMATIC REG screens), added as a solid powder to the coating melt. Thecoverage of BaSO₄.Sr,Eu was 0.65 g/m².

Example 5

Single layer coating Examples 5 is identical to Example 4 except thecoverage of BaSO₄.Sr,Eu was 1.30 μg/m².

Example 6

Example 6 demonstrates the sensitivity of coating Examples 4-5 to x-rayexposure. For comparison, a sample of coating Example 3 was alsocombined with an external green light emitting screen (LaPO₄:Ce,Th at 60g/m², emitting at 544 nm); another sample was combined with an externalblue light emitting screen (BaSO₄.Sr,Eu at 60 g/m², emitting at 394 nm).Coating Example 3 was placed in face side contact with each screen andthe combined packet was exposed with the backside of the screen facingthe x-ray source. Samples of coating Example 4 and 5 were exposeddirectly without a screen. The x-ray exposures ranged from 1.02 mRems to302 mRems. Table 7 below lists the source settings for each exposurelevel, and Table 8 shows the developed blue transmission density afterprocessing each sample strip at 162° C. for 30 sec. TABLE 7 RelativeMeasured Distance Cu* Al LUCITE exposure mRems mA mSec kVp (cm) mAslog(E) mirror (mm) (mm) (cm) 1 0.92 200 200 60 177.8 40 −0.04 no 0.5 15.08 4 3.63 510 300 60 177.8 153 0.56 no 0.5 1 5.08 16 14.17 570 300 6097.8 171 1.15 no 0.5 1 5.08 32 28.2 570 600 60 97.8 342 1.45 no 0.5 15.08 64 56.5 570 1200 60 97.8 684 1.75 no 0.5 1 5.08 128 125.1 110 14060 97.8 15.4 2.10 no 0 0 0 300 301 650 370 60 97.8 240.5 2.48 no 0 0 0

TABLE 8 Coating Example 3 Example 4 Example 5 Example 3 InternalPhosphor None BaSO₄.Sr, Eu BaSO₄.Sr, Eu None (blue) 0.65 g/m² 1.30 g/m²External Phosphor Blue (394 nm) none none (Green 544 nm) Screen(XOMATIC) LaPO₄: Ce, Tb 60 g/m² Measured mRems log(E) Blue TransmissionDensity 0.92 −0.04 1.066 1.076 1.149 1.540 3.63 0.56 1.104 1.075 1.2341.536 14.17 1.15 1.022 1.099 1.224 1.442 28.2 1.45 1.098 1.032 1.2141.295 56.5 1.75 0.970 1.059 1.204 1.320 125.1 2.10 0.860 1.033 1.1240.840 301 2.48 0.855 0.605 0.520 0.822 Delta Density 0.209 0.478 0.6820.684Delta density = (density at 0.92 mRem − density at 301 mRem)

The delta densities indicate that an embedded phosphor in thephotothermgraphic element provides an effective recording medium fordirect x-ray exposures and is dependent on phosphor coverage.

Example 7

Example 7 illustrates the use of coating Example 1 for making dentalexposures and a digital process for making prints. Coating Example 1 wascombined in face side contact with a blue emitting phosphor screen(DUPONT PAR AA 507026F). The backside of the screen faced the x-raysource. A cadaver dental object was placed on top of the film packet,and the object and was given a 50 mRem exposure. Coating Example 1 wasprocessed at 162° C. for 24 sec to produce a positive image of the teethin the target. The positive image in the film was digitally scanned intoAdobe Photoshop, then grayscaled, contrast adjusted, and finally savedas both negative and positive image files. Both negative and positiveimage files were diagnostically useful images. Obtaining a positiveimage was advantageous because certain pathologies are more visible thanin the corresponding negative image.

The invention has been described in detail with particular reference topreferred embodiments, but it will be understood that variations andmodifications can be effected within the spirit and scope of theinvention.

1. A method of forming a positive image in a photothermographic assemblycomprising a photothermographic material and an intensifying means forconverting ionizing radiation, wherein the assembly has been imagewiseexposed to ionizing radiation to form a latent image in thephotothermographic material, which photothermographic material has atleast one imaging layer comprising a potentially negative-workingemulsion, said method further comprising thermally developing theimagewise exposed assembly wherein thermal development of unexposedsilver salts in exposed areas is effectively inhibited relative tounexposed areas, thereby producing a positive image.
 2. The method ofclaim 1, further comprising a developer or precursor thereof and anoxidized developer scavenging agent to accelerate development byremoving oxidized developer as it is formed during the thermaldevelopment.
 3. The method of claim 1 wherein the intensifying means andphotothermographic material comprise separate elements in the assembly.4. The method of claim 1 which method comprises imagewise exposing thephotothermographic material with a non-solarizing amount of radiation orenergy to form a latent image and completely developing the latent imageto a positive image in a single thermal development unit step to producea positive image in the photothermographic material.
 5. The method ofclaim 1, wherein the photothermographic material forms a positive imageat high speed when exposed and heated 10 to 40 sec at 150 to 200° C.,wherein the ISO speed is at least ISO 100 and as high as ISO
 24000. 6.The method of claim 1 wherein the thermal development of unexposedsilver salts in the exposed areas is inhibited relative to the unexposedareas by a density inhibiting agent.
 7. The method of claim 6 whereinthe density-inhibiting agent is released by a precursor compound duringthe thermal development.
 8. The method of claim 1 wherein thephotothermographic material comprises a silver-halide emulsion, in whichsilver-halide grains are spectrally sensitized to light wavelengths in arange 350 nm to 1500 nm, and at least one non-light-sensitive organicsilver salt, said method comprising, following thermal development ofthe imagewise exposed material, forming imagewise reduced silver that isphysically separate and morphologically distinct from a developedlatent-image silver associated with the silver-halide grains.
 9. Themethod of claim 1 comprising, following the thermal development, thefollowing steps: scanning the developed positive image to form an analogelectronic representation of the developed image; digitizing an analogelectronic representation to form a digital image; digitally modifyingthe digital image; and storing, transmitting, printing, or displayingthe modified digital image.
 10. The method of claim 1, wherein thephotothermographic material is a high speed black-and-white film. 11.The method of claim 1 wherein the potentially negative-working emulsioncomprises primarily tabular grains.
 12. The method of claim 1, whereinthe photothermographic material comprises at least one light-sensitiveimaging layer comprising a potentially negative-working emulsion thatcomprises light-sensitive silver halide, one or more non-light-sensitiveorganic silver salts, and wherein the photothermographic material isthermally developed without any externally applied developing agent byheating the photothermographic material in a thermal processor to atemperature greater than 150° C. in an essentially dry process to form apositive image in the photothermographic imaging layer, said methodfurther comprising scanning the positive image to provide a digitalelectronic record capable of generating a positive or a negative imagein a display element.
 13. The method of claim 12 wherein thephotothermographic material further comprises a developing agent orprecursor thereof and an effective amount of a Dox scavenger forremoving oxidized developer as it is being formed during thermaldevelopment.
 14. The method of claim 1 wherein the intensifying means isa phosphor that emits visible light upon exposure to ionizing radiation.15. The method of claim 1 wherein the intensifying means is a metal foilthat emits lower energy beta particles upon exposure to ionizingradiation.
 16. The method of claim 1 wherein the intensifying means is aphosphor screen wherein phosphor particles or amorphous scintillatorparticles are dispersed in a polymeric binder solution then coated on asupport to form a fluorescent layer that upon irradiation with ionizingradiation can be used to imagewise expose the at least one imaginglayer.
 17. The method of claim 1 wherein the intensifying means is anx-ray sensitive phosphor layer in combination with a photocathode thatemits photoelectrons in response to exposure to ionizing radiationwherein the photoelectrons are accelerated by an external applied fieldto bombard a second phosphor screen where the visible light emissionfrom the second phosphor screen is used for the purpose of exposing thephotothermographic material to form a latent image therein.
 18. Themethod of claim 1 wherein the intensifying means is dispersed in thepotentially negative-working emulsion.
 19. The method of claim 1 whereinthe photothermographic assembly is placed in a light-tight package. 20.The method of claim 19 wherein the photothermographic assembly is anintra-oral dental film packet.
 21. The method of claim 19 wherein thelight-tight package comprises a liner made of a material that will nottransmit visible light in a light-tight package.
 22. Aphotothermographic assembly comprising a photothermographic material andan intensifying means for converting ionizing radiation, wherein theassembly has been imagewise exposed to ionizing radiation to form animagewise exposed and developed photothermographic material comprising:(a) prior to imagewise exposure and development, at least one imageforming layer coated on a support, the at least one image forming layercomprising at least one photographically active silver-halide emulsionand at least one non-light-sensitive organic silver salt, (b) followingimagewise exposure and development, an imagewise reduced silver that isphysically separate and morphologically distinct from developed latentimage silver associated with silver-halide grains.
 23. Thephotothermographic assembly of claim 22 wherein the at least onephotographically active silver-halide emulsion further comprises abinder, a Dox scavenger, a developer or developer precursor, and adensity-inhibiting agent or precursor thereof, wherein upon thermaldevelopment, the ratio of density produced in an unexposed area todensity produced in a highest exposed area is greater than 1.1.
 24. Thephotothermographic assembly of claim 23 wherein the developer is anamine developer or precursor thereof.
 25. The photothermographicassembly of claim 23, wherein the density-inhibiting agent, during thethermal development, inhibits development of unexposed silver particlesin the exposed areas relative to the unexposed areas.
 26. Thephotothermographic assembly of claim 22, wherein the photothermographicmaterial is capable of forming a high-speed direct-positive image afterfull development that is at least two stops faster than a low-contrastthermally developed negative image.
 27. The photothermographic assemblyof claim 25, wherein the density-inhibiting agent comprises amercapto-functional compound.